View source for Lymphopoiesis

'''Lymphopoiesis''' (lĭm'fō-poi-ē'sĭs) refers to the generation of [[lymphocyte]]s, one of the five different types of [[white blood cells]] (WBC), and is also more formally called [[lymphoid]] [[hematopoiesis]]. 

{{Lymphopoiesis Glossary}}
==The name Lymphopoiesis==

[[Lymphocytes]] are considered to be of the [[lymphoid]] lineage as opposed to other lineages of blood cells such as the [[myeloid]] lineage and the [[erythroid]] lineage. 
[[Image:Illya Mechnikov.JPG|thumb|right|Immunology pioneer [[Elie Metchnikoff]]]]

Nomenclature, the problem of naming things properly, is not trivial in this case because lymphocytes - which are certainly found in blood and originate along with blood cells in the bone marrow of the adult - are almost by definition also strongly associated with lymph fluid and a separate connective system, the lymphatic system, which is assuredly not the blood system. Connected to, parallel to and interacting with and feeding into the blood system, yes, but easily distinguished apart by any scientific observer.

Lymphopoiesis is now used interchangeably with the term "lymphocytopoiesis" - the making of lymphocytes - but other sources may distinguish between the two, stating that "lymphopoiesis" additionally refers to creating [[lymphatic tissue]], while "lymphocytopoiesis" refers only to the creation of cells in that tissue. It is rare now for lymphopoiesis to refer to the creation of lymphatic tissues. 

[[Myelopoiesis]] refers to 'generation of cells of the myeloid lineage' and [[erythropoiesis]] refers to 'generation of cells of the erythroid lineage' etc., so parallel usage has evolved in which lymphopoiesis refers to 'generation of cells of the lymphoid lineage'.

Observations on research going back well over 100 years had elucidated the two great classes of WBC - Myeloid and Lymphoid - and great advances in medicine and science have resulted from these studies. It was only natural to ask where these two great classes of cells arose, and after much work two cell types with some strong [[stem cell]] properties were isolated and defined - CMP, the [[common myeloid progenitor]] and CLP, the common lymphoid progenitor for mice. But science is an additive game and it was eventually found these [[progenitor]]s were not unique, and further that the two great families of Myeloid and Lymphoid were not disjoint, but rather two partially interwoven family trees.  This is more than just nomenclature, it is new science that provides challenges of complexity yet offers new vistas of bio-science and the promise of early enhancement of private and public health issues. And it gives insight into the nature of redundancy and overlap in the immune system and hints how to use this to advantage.

==The purpose of Lymphopoiesis==

The complete loss of or loss of function of any WBC cell type is a serious health matter, but lymphopoiesis is absolutely necessary for life. Mature lymphocytes are a critical part of the immune system that (with the exception of memory B and T cells) have short lives measured in days or weeks and must be continuously generated throughout life by cell division and differentiation from cells such as common lymphoid [[Progenitor cell|progenitors]] (CLPs) in mice. Were this system to fail, the body would be largely undefended from infection. 

The set comprising CLP cells and similar progenitors are themselves descendants of the [[pluripotential hemopoietic stem cell]] (pHSC) which is capable of generating all of the cell types of the complete blood cell system.<ref>''Stem Cell Biology'', page 307</ref> Despite their remarkable ability to generate the complete suite of lymphocytes, most progenitors are not true stem cells, however, and must be continually renewed by differentiation from the pHSC stem cell. 

Many progenitor cells are also referred to as ''transit cells'',<ref>''Dynamics of Cancer'', page 251</ref><ref>[http://www.jrank.org/health/pages/20765/transit-cells.html transit cells - q.v., Also called]</ref>, sometimes also called ''transit amplifying cells'', the meaning of this term being that the transit cell may found a new sub-lineage but the number of resultant cells is strictly limited (although possibly very large, even trillions yet finite) and the lineage is terminated by cells that die off (by [[apoptosis]]) or remain as cells that can no longer divide. Examples of such cells are CFUs (Colony-forming units - referred to as such because of their ability to form colonies in vitro in artificial media) such as CFU-T<ref>[http://www.copewithcytokines.de/cope.cgi?key=CFU-T  CFU-T] </ref>.

In mice, transplantation of a single pHSC cell can reconstitute a sub-lethally irradiated host (i.e. a mouse that has been irradiated so that all leukocytes are killed) with all these lineages of cells, including all types of lymphocytes via CLPs. This has been known for more than 40 years.

Lymphopoiesis continues throughout life and so progenitor cells and their parent stem cells must always be present.

==Overview of Lymphopoiesis==

In the case of mammals such as man lymphopoiesis begins with limited passive provision by the mother of lymphocytes and substantial immunoglobulin G that cross the [[placenta]] and enter the fetus to provide some protection against [[pathogen]]s, and also leukocytes that come from breast milk and enter circulation via the digestive tract.<ref>[http://www.ncbi.nlm.nih.gov/pubmed/18394715  Transfer of maternal colostral leukocytes promotes development of the neonatal immune system] </ref>
 
However early in [[gestation]] the developing embryo has begun its own lymphopoiesis from the [[fetal liver]]. Lymphopoiesis also arises from the [[yolk sac]]<ref>Adult T-cell Progenitors Retain Myeloid Potential; ''Nature''; 2008</ref>. This is in contrast to the adult where all lymphocytes originate in the bone marrow.<ref>''Textbook of Human Development and Histology'', p.176</ref>

There are four major types of lymphocytes, many sub-types, and hundreds or thousands of lymphocyte cell types that have been identified by scientists. All are generated by normal or abnormal lymphopoiesis except for certain artificial strains created in the laboratory by development from existing strains. Although lymphocytes are usually considered mature (as seen in blood tests) they are certainly not inert but can and do get around the body to anywhere there is a need; and when such need arises, new rounds of 'downstream' lymphopoiesis such as cell multiplication and differentiation may arise, coupled with intense [[mitotic]] and [[metabolic activity]].

This is hardly a simple topic. In his 1976 text ''Immunology, Aging and Cancer'' immunologist and Nobel Prize winner Sir Frank Macfarlane Burnet speculated that the immune system might one day be found to be as complex as the nervous system. As the production of lymphocytes is so close to the central role of the [[immune response]] it is wise to approach the study of it with some humility in the face of the task, although there are general principles that help in understanding.

==The process of Lymphopoiesis==

{| align=right border=1 width=350 cellpadding=2 cellspacing=0 class=toccolours style="margin-left:0.5em;"
! align=center style="background:#CCBBEE" |Lymphopoiesis Acronyms
|-
| • '''B-NK''' Progenitor for B and NK
|-
| • '''CB'''  Cord blood
|-
| • '''CFU''' Colony-forming Unit
|-
| • '''CLP'''  Common Lymphoid Progenitor
|-
| • '''CMP'''  Common Myeloid Progenitor
|-
| • '''DC'''  Dendritic Cell (Myeloid or Lymphoid)
|-
| • '''ELP'''  Early Lymphoid Progenitors
|- 
| • '''ETP'''  the most primitive cells in the thymus are the Early Thymocyte Progenitors 
|-
| • '''G-CSF''' Granulocyte Colony Stimulating Factor
|-
| • '''GM-CSF''' Granulocyte Macrophage Colony Stimulating Factor
|-
| • '''GMP'''  Granulocyte Macrophage Progenitor;
|-
| • '''HSC'''  pluripotential Hemopoietic Stem Cell 
|-
| • '''MDC'''  combined Macrophage and DC progenitor potential 
|-
| • '''MEP'''  megakaryocytic and erythroid progenitor 
|-
| • '''MLP'''  Multi-lymphoid Progenitor potential, any progenitor minimally able to give rise to B cells, T cells and NK cells 
|-
| • '''MPP'''  Multipotent Progenitor
|-
| • '''Notch'''  [[Notch signaling pathway]] re T Cell commitment from progenitors
|-
| align=right | <small class="editlink noprint plainlinksneverexpand">[{{SERVER}}{{localurl:Template:Lymphopoiesis  acronyms|action=edit}} edit ]</small>
|}

Lymphopoiesis can be viewed in a ''mathematical'' sense as a recursive process of cell division and also as a process of differentiation, measured by changes to the properties of cells.

*Given that lymphocytes arise from specific types of limited stem cells - which we can call P (for Progenitor) cells - such cells can divide in several ways. These are general principles of limited stem cells.

Considering the P as the ‘mother’ cell, but not a true stem cell, it may divide into two new cells, which are themselves identical, but differ to some degree from the mother. Or the mother cell P may divide unequally into two new daughter cells both of which differ from each other and also from the mother. 

Any daughter cell will usually have new specialized abilities and if it is able to divide it will form a new sub-lineage. The difference of a daughter cell from the mother may be great, but it could also be much less, even subtle. What the P mother cell does ''not'' do is divide into two new P mother cells or a mother and a daughter; this is a matter of observation as such limited progenitor cells are known to not self-renew.

*There is a sort of exception when daughter cells at some level of the lineage may divide several times to form more seemingly identical cells, but then further differentiation and division will inevitably occur, until a final stage is reached in which no further division can occur and the cell type lineage is finally mature. An example of maturity is a plasma cell, from the B cell lineage, which produces copious antibody, but cannot divide and eventually dies after a few days or weeks.

*The progenitor CLP of the mouse or the progenitor MLP of the human differentiates into lymphocytes by first becoming a lymphoblast (Medical Immunology, p. 10). It then divides several more times to become a prolymphocyte that has specific cell-surface markers unique to either a (1) [[T cell]] or (2) [[B cell]]. The progenitor can also differentiate into (3) [[natural killer cell]]s (NK) and (4) [[dendritic cell]]s.

*T Cells, B Cells and NK Cells are unique to the lymphocyte family, but dendritic cells are not. Dendritic cells of identical appearance but different markers are spread throughout the body, and come from either lymphoid and myeloid lineages, but these cells may have somewhat different tasks and may take up lodging preferentially in different locations.<ref>''Immuno-Biology, The Immune System in Health and Science''.</ref>  (Revise in light of new research) This is now an open question; also, the different dendritic cell lineages may have different ‘tasks’ and stay in different ‘locations.’ <ref>''Fundamental Immunology'' 5th edition</ref>

T and B lymphocytes are indistinguishable [[Histology|histologically]]（that is, under a light microscope they cannot be told apart.）Indeed, the inactive B and T cells are so featureless with few cytoplasmic [[organelle]]s and mostly inactive [[chromatin]] that until the 1960s textbooks could describe these cells, now the central focus of immunology, as having no known function!!<ref>''Immuno-Biology, The Immune System in Health and Science''. Garland Science Publishing</ref> 

However T and B lymphocytes are very distinct cell lineages and they ‘grow up’ in different places in the body. They perform quite different (although co-operative) functions in the body. No evidence has ever been found that T and B cells can ever interconvert. T and B cells are [[biochemically]] distinct and this is reflected in the differing markers and receptors they possess on their cell surfaces. This seems to be true in all vertebrates, although there are many differences in the details between the species.

*Regardless of whether the CLP (mouse) or MLP or a small closely related set of progenitor cells take credit for generating the profusion of lymphocytes, it remains an interesting observation that the same lymphoid progenitors can still opt to generate some cells that are clearly identifiably myeloid.

===Lymphopoiesis for T cells===

T cells are formed in [[bone marrow]] then migrate to the cortex of the [[thymus]] to undergo maturation in an antigen-free environment for about one week where a mere 2-4% of the T cells succeed. The remaining 96-98% of T cells die by [[apoptosis]] and are [[phagocytosed]] by [[macrophages]] in the thymus. So many thymocytes (T cells) die during the maturation process because there is intensive screening to make sure each thymocyte has the ability to recognize [[major histocompatibility complex|self-peptide:self-MHC complex]] {{Citation needed|date=July 2010}} and for [[Self_tolerance#Innate_tolerance|self tolerance]]. The apoptosed thymocyte dies a willing and noble death and it is quickly recycled.

Upon maturity, there are several forms of thymocytes including <ref>''Textbook of Medical Immunology'', page 5</ref>
* [[T helper cell|T-helper]] (needed for activation of other cells such as B cells and macrophages), 
* [[Cytotoxic T cell|T-cytotoxic]] (which kill virally-infected cells), 
* [[Memory T cell|T-memory]] (T cells that remember antigens previously encountered), and
*[[Regulatory T cell|T-suppressor cells]] (which moderate the immune response of other leukocytes). Also called T-regulatory Cell, or just ‘Treg(s)’, to be cool.

When T-Cells become [[Immunologic activation|activated]] they undergo a further series of developments. A small, resting T lymphocyte rapidly undergoes [[blastogenic transformation]] into a large lymphocyte (13–15μm).  This large lymphocyte (known in this context as a [[lymphoblast]]) then divides several times to produce an expanded population of medium (9–12μm) and small lymphocytes (5–8μm) with the same  [[Antigen#Antigenic_Specificity|antigenic specificity]].<ref>''Medical Immunology'', page 23</ref> Final activated and differentiated T lymphocytes are once again [[Morphology (biology)|morphologically]] indistinguishable from a small, resting lymphocyte. Thus the following developmental states may be noticed in sequence in blood tests:
# Prolymphocyte
# Large lymphocyte
# Small lymphocyte

'''Basic Map of T Cell lymphopoiesis'''

This basic map of T Cell formation, in sequence, is simplified and is akin to textbook descriptions, and may not reflect latest research.(Medical Immunology, p.119)

In the thymus
*MLP
*ETP
*DN1
*(B; Mφ)
*DN2
**(DC; NK)
*DN3
**(γδ)
*DN4
*DP
**(TNK; CD4; CD8; Treg)

In the [[Periphery]]
*(Th1; Th2)

====More details on T Lymphopoiesis====

Unlike other lymphoid lineages, T cell development occurs almost exclusively in the thymus.  T-lymphopoiesis does not occur automatically but  requires signals generated from the thymic [[stromal]] cells. The process has an astonishingly complex beauty to it. Several stages at which specific regulators and growth factors are required for T cell development to proceed have been defined. Interestingly, later in T cell development and its maturation these same regulatory factors again are used to influence T cell specialization. 

T cells are unique among the lymphocyte populations in their ability to further specialize as mature cells and become yet more mature. And T Cells come in many flavors, for example: the conventional TcRαβ T cells; the so-called unconventional TcRγδ T cells; NKT cells; and T regulatory cells (Treg).  Details regarding the developmental and life cycle of the unconventional T cells are less well-described compared to the conventional T cells.

'''Stages of T cell maturation'''

<u>Stage One:</u> Thymic Migration

Multipotent lymphoid progenitors (MLP) enter the T cell pathway as they immigrate to the thymus. The most primitive cells in the thymus are the early thymocyte progenitors (ETP), which retain all lymphoid and myeloid potential but exist only transiently, rapidly differentiating into T and NK lineages. (Medical Immunology, p. 118)

<u>Stage Two:</u> Proliferative Expansion and T Lineage Commitment

Final commitment to the T cell lineage occurs within the thymic microenvironment, the microscopic structures of the thymus where T cells are nurtured. The most primitive T cells retain pluripotential ability and can differentiate into cells of the myeloid or lymphoid lineages (B cells, DC, T cells, or NK cells).

More differentiated double negative T cells (DN2 cells) have more limited potentiality but are not yet fully restricted to the T cell lineage (they can still develop into DC, T cells, or NK cells). Later on, they are fully committed to the T cell lineage- when thymoctyes expressing Notch1 receptors engage thymic stromal cells expressing Notch1 ligands, the thymocytes become finally committed to the T-cell lineage. See Gallery Image “Double Negatives”

With the commitment to the T cell lineage, begins a very complex process known as [[T_cell_receptor |TcR]] gene rearrangement. This creates an enormous diversity of T cells bearing antigen receptors. Afterward some T cells leave the thymus to migrate to the skin and [[mucosae]].

<u>Stage Three:</u> β-Selection

<u>Stage Four:</u> T Cell Receptors Selection

Only 2% to 3% of the differentiating thymocytes, those that express TcR capable of interaction with MHC molecules, but tolerant to self-peptides, {{explain}} survive the Stage Four selection process. 

<u>Stage Five:</u> Continuing Differentiation in the Periphery

It was previously believed that the human thymus remained active as the site of T cell differentiation only until early adulthood and that later in adult life the thymus [[atrophies]], perhaps even vanishing. Recent reports indicate that the human thymus is active throughout adult life. Thus several factors may contribute to the supply of T cells in adult life: generation in the thymus, extra-thymic differentiation, and the fact that memory T cells are long-lived and survive for decades.

====Even more details====

*Unconventional T cells
The thymus also gives rise to the so-called ‘unconventional T cells’ such at γδ T cells, Natural Killer T cells (NKT) and regulatory T cells (Treg).

*γδ T cells
γδT cells  represent only 1% to 5% of the circulating T cells, but are abundant in the mucosal immune system and the skin, where  they represent the dominant T cell population. These ‘non-MHC restricted T cells’ are involved in specific primary immune responses, tumor surveillance, immune regulation and wound healing.{{Citation needed|date=August 2010}}

Several differences between αβ and γδ T cell development have been described. They emigrate from the thymus in “waves” of clonal populations, which home to discrete tissues. For example, one kind is found in the peripheral blood while another predominates in the intestinal tract.

*Natural Killer T cells
Human NKT cells are a unique population and are thought to play an important role in tumor immunity<ref>[http://www.nccr-oncology.ch/Lay_Summaries_about_the_Programme_and_its_Projects/Tumor_Immunity_and_Cancer_Immunotherapy.html  Tumor Immunity and Cancer Immunotherapy] </ref>  and [[Immunoregulator|immunoregulation]].

*T Regulatory cells
"Tregs" are considered as naturally occurring regulatory T cells. Tregs comprised about 5% of the circulating [[CD4%2B |CD4+]] T cells. These cells are thought to possess important an autoimmunity property by regulating 'autoreactive' T cells in the [[periphery]]. (Medical Immunology, p. 117-122)

===Lymphopoiesis for B cells===

B cells are formed and mature in bone marrow (and spleen). 

It is a good mnemonic aide that B cells are formed in the bone marrow, but it is a mere coincidence since B cells were first studied in the chicken's [[bursa of Fabricius]] and it is from this bursa that B cells get their name.

These B cells then leave the bone marrow and migrate to peripheral lymphoid tissues, such as a lymph node. Once in a secondary lymphoid organ the B cell can be introduced to an antigen that it is able to recognize. 

Through this antigen recognition and other cell interactions the B cell becomes activated and then divides and differentiates to become a plasma cell. The plasma cell, a B cell end product, is a very active [[antibody]]-secreting cell that helps protect the body by attacking and binding to antigen. 

Even after many decades of research, some controversy remains as to where B cells mature and 'complete their education', with the possibility remaining that the site may also partially be [[Gut-associated lymphoid tissue|peri-intestinal lymphoid tissues]].<ref>''Medical Immunology'', page 22</ref>

B lymphopoiesis occurs exclusively in the bone marrow and B lymphocytes are made continuously throughout life there in a 'microenvironment' composed of [[stromal cells]], [[extracellular matrix]], cytokines and growth factors, which are critical for proliferation, differentiation, and survival of early lymphocyte and B-lineage precursors. 

The relative proportion of precursor B cells in the bone marrow remains rather constant throughout the life span of an organism. There are stages such as Pre-B-I cells (5% to 10% of the total); Pre-B-II cells (60% to 70%) while the remaining 20% to 25% are immature B cells.  Most textbooks say that B Cells mature in the bone marrow but, generally, immature B cells migrate to the spleen for 'higher education' of some sort where they go through  transitional stages before final maturation.(Medical Immunology, p. 136)

B lymphocytes are identified by the presence of soluble [[immunoglobulin G]] (IgG). This is the most common protective immunoglobulin in the adult body. After antigenic stimulation, B cells differentiate into plasma cells that secrete large quantities of soluble IgG. This is the final stage of B lymphopoiesis but it is the clincher because the plasma cells must either issue antibody close to a source of infection, or disseminate it in the blood to fight an infection at a distance or in an inaccessible part of the body.

'''Basic map of B cell lymphopoiesis'''

A generally regarded valid map of B cell lymphopoiesis is as follows in sequence, in two parts with the first being in the bone marrow and the second in the spleen:<ref>''Medical Immunology'', p. 123</ref>. The development process in the bone marrow occurs in [[Germinal center|Germinal Centers]].

In the bone marrow
*Pro-B
*Pre-B-I
*Pre-B-II large
*Pre-B-II small
*Imm(ature)

In the spleen
*T1
*T2/T3
*(Marginal Zone (MZ); B-1 ; B-2)
* B-2 further differentiate into:
**(Germinal Center (GC); Memory ; Plasma )

===Lymphopoiesis for NK cells===

NK cells, which lack antigen specific receptors, develop in the bone marrow. After maturation and release from the marrow they circulate in the blood through their lifetime seeking opportunity. The opportunity they seek is to encounter and recognize and then kill abnormal cells such as [[cancer]] or virally-infected cells. It is well-known that lymphocytes never have granules  or at least not granules that are readily visible even upon [[staining]]. Everyone knows this, but NK cells are the exception. They do have numerous granules which provide their ability to [[Natural_killer_cell#Mechanism|kill cells]] and these granules are why NK cells have an alternate name, LGL, Large Granular Lymphocytes.

NK cells not only have a catchy movie-title name (Natural Killer) but are also the only lymphocytes considered part of the [[innate immune system]] (in contrast to the [[adaptive immune system]]. Yet they are much more closely related to T cells (part of the [[adaptive immune system]]) than to other cells of the innate immune system. NK cells not only share many surface markers, functions and activities in common  with  T  Cells, they also  arise  from  a common T/NK progenitor.  The T/NK  precursor is also believed to be the source of a subpopulation of lymphoid DC. (Medical Immunology, p. 121)

NK cells have a definition 'barcode' as CD3, CD16+, CD56t lymphocytes. (See Barcode Section of this article). NK progenitors can be found mainly in the thymus (mouse), but the thymus is not absolutely required for NK development.  Probably NK cells can develop in a variety of organs but the major site of NK cell development is not known.

In humans, the majority (85–90%) of the NK cells have a high cytolytic capacity (the ability to [[lysis|lyse]] cells). A smaller subset (10–15%) called NK 'CD56 bright' is chiefly responsible for [[cytokine]] production and has [[enhanced survival]]. Traveling to [[lymph nodes]] the 'CD56 bright' NK cells differentiate again into mature NK cells which express [[Killer-cell_immunoglobulin-like_receptor | killer cell immunoglobulin-like receptors]] (KIR), natural cytotoxicity receptors (NCR), and critical [[adhesion molecules]]. (Medical Immunology, p. 122)

===Lymphopoiesis for dendritic cells===

''Dendritic cell'' is usually abbreviated DC or DCs. The process by which CLP cells may differentiate to generate [[Dendritic_cell#Formation_of_immature_cells|dendritic cells]] of lymphoid lineage is not yet well defined.<ref>''Fundamental Immunology''; Paul; Ch. 15 "DISTRIBUTION OF DENDRITIC CELLS IN VIVO: A MULTIMEMBER FAMILY"</ref>

DCs are highly specialized and efficient [[antigen-presenting cells]]. Cells identical in appearance come both from a myeloid lineage (referred to as myeloid dendritic cells) and also from a lymphoid lineage (referred to as [[plasmacytoid]] dendritic cells).

The development and regulation of DC is not well-characterized. While the DC precursors have been identified in the human fetal liver, thymus, and bone marrow, during adult life DC are thought to be produced only from the bone marrow and released into the blood to wander and settle down. Overall a large number of DC of varying types are dispatched throughout the body, especially at epithelia such as skin, to monitor invaders and nibble their antigens. (Medical Immunology, p.122)

===Comparison of Killers from Lymphopoiesis===

Lymphocytes have a number of alarming properties such as the ability to wander around the body and take up lodging almost anywhere, and while on the way issue commands in the form of [[cytokines]] and [[chemokines]] and [[lymphokines]], commands that affect many cell types in the body and which may also recursively induce further lymphopoiesis. One strong behavior pattern that captivates researchers and the public alike is the ability of lymphocytes to act as police, judge and executioner to kill other cells or demand that they suicide, a command that is usually obeyed. There seems to be no other sentencing option available.

Killers are distinguished from cells such as macrophages that eat other cells or munch debris by a method called [[phagocytosis]]. Killers do not use phagocytosis, they just kill and leave the clean-up to other cells.

Killers are known to attack virus-infected cells and cells that have become cancerous. Because of these abilities much research has been done into transforming these qualities into medical therapy but progress has been slow.  

Here is the parade of killers and how they work:

*Cytotoxic T cells 
(also called '''Tc''' or antigen-specific cytolytic T lymphocytes ('''CTL)''') <ref>''Immunology''; Lydyard et al; p. 22, 132-137</ref>. Tc kill by apoptosis and either splash their target with [[perforin]] or [[granzymes]] or else use [[Fas-Fasl Interaction]] to command target elimination. This kills cells that are infected and display antigen.

*NK cells (also called LGL (large granular lymphocytes))
These kill with exactly the same methods as Tc, but have no interaction with any antigen. They select their targets based on typical molecules displayed by cells that are under stress by viral infection. NK Cells mainly are in the circulation (5-15% of the circulating lymphocytes) yet are also distributed in tissues everywhere.<ref>''Immunology''; Lydyard et al; p. 15, 18-20,41</ref>

*LAK cells (Lymphokine-activated killer) are a laboratory/clinical subset of NK Cells promoted by [[IL-2]] to attack tumor cells.<ref>''Immunology''; Lydyard et al; p. 20, 259-260</ref>

*NKT cells  see [[Natural Killer T cell]] main article
Natural Killer T Cells. Human NK T cells are a unique population (which express NK cell markers such as CD56 and KIR). NKT cells are thought to play an important role in tumor immunity and immunoregulation. (Medical Immunology, p.135), yet little is known. Recent evidence suggests a role working together with [[Hepatic stellate cell|hepatic stellate cells]] being a liver-resident antigen-presenting cell that presents lipid antigens to and stimulates proliferation of NKT cells.

*Natural killer-like T cells
A heterogeneous group with ill-defined properties.

However in summary there is ''no known cell or set of cells'' that is capable of killing cancerous cells in general.

===Labeling Lymphopoiesis===
    
Because all WBCs are microscopic, colorless and often seemingly identical in appearance they are individually identified by their natural chemical markers, many of which have been analyzed and named. When two cells have the same markers, the reasonable assumption is made that the cells are identical at that time.  A set of markers is colloquially describes as the ''barcode'' for that cell or that cell line.

*Here is an example of how a barcode can come to be, for the all-important HSC as an example.

HSC are technically described as: lacking FMS-like tyrosine kinase3 (Flt3) and lacking the markers specific to discrete lymphoid lineages (Lin), but expressing high levels of Sca1 and c-kit; HSC also express CD44, low levels of Thy1.1 (CD90), but no IL-7Ra or CD27. 
      
This is called the (surface) phenotype of an HSC. It can be expressed as a set (Lin2, Sca1high, c-kit high, CD44+, Thy1.1low, CD27 2, and IL-7Ra2). This set is a ‘barcode’ for the HSC, akin to the barcode label attached to your chicken-wing plastic bag for checkout at a supermarket! Scientists use these barcodes to check, categorize and accumulate cells for many purposes often using laboratory methods such as [[Flow cytometry|cell flow cytometry]]. These barcodes partially define the modern meaning of [[phenotype]] for [[leukocytes]].
   
Progression of HSC differentiation and lineage commitment is indicated by changes in this [[phenotype]]. That is, as the cell changes, the markers will also change and the barcode will change.

*Typical barcodes for some cell types appearing in this article.

{|border="1" cellpadding="5"
|-
!Cell Type
!Barcode
|-
|ETP 
|C-Kit+, CD44+, CD25-
|-
|DN1	
|CD44+, CD25-
|-
|DN2
|CD44+, CD25+
|}

*Note explaining the barcode parameter details: ''Flt3 is a cytokine tyrosine kinase receptor thought to be important in early lymphoid development. In addition, Flt3 plays a major role in maintaining B lymphoid progenitors. CD27 plays a role in lymphoid proliferation, differentiation, and apoptosis. The acquisition of CD27 and Flt3 by the HSC coincides with the loss of long-term repopulating potential. At this stage the cells retain both lymphoid and myeloid potential and are referred to as multipotent progenitors.'' (Medical Immunology, p. 114)

===Developing our knowledge of Lymphopoiesis===

Remember that immunology is a developing field. New questions emerge in immunology continuously as though there were a [[stem cell]] for questions. For example it was thought that the process of lymphopoiesis was a direct, orderly unidirectional sequence. But it is not clear  if end-stage lymphocytes come from progenitors that are homogeneous populations or overlapping populations. Nor is it clear whether lineages of lymphocytes develop via a continuum of differentiation with a progressive loss of lineage options or whether abrupt events result in the acquisition of certain properties.<ref>''Medical Immunology''; Litwin, p. 122</ref>   

Changes in cytoplasm, [[Morphology (biology)|morphology]] of the cell nucleus, granules, cell internal biochemistry, signaling molecules and cell surface markers are difficult to correlate with definite stages in lymphopoiesis. The morphological differences don't just correspond to steps in [[mitosis]] (somatic cell division), but result from continuous “maturation processes”of the cell nucleus as well as of the cytoplasm and so one must not be too rigid about morphological distinctions between certain cell stages.<ref>''Color Atlas of Hematology'' 2004</ref>

*Models and updates on the Lymphopoiesis family tree

Until recently the model of the CMP generating all myeloid cell and the CLP generating all lymphoid cells was considered necessary and sufficient to explain the known facts observed in the generation of WBCs, and it is still found in most basic textbooks. However beginning around 2000 and gaining momentum after 2005<ref>"The Earliest Thymic Progenitors for T cells Possess Myeloid Lineage Potential"; Bell, Bhandoola; Vol 452, 10 April 2008, doi:10.1038/nature06840</ref> in both studies on man and mouse, new complexities were noted and published in papers.{{Citation needed|date=August 2010}} These studies are important now mainly to immunology researchers but are likely to eventually lead to changes in medical treatments.

The changes were sparked by observations that lymphopoiesis did not always break into two lineages at the level of the CLP. Worse, some macrophages (long considered a myeloid lineage) could be generated by lymphoid lineage progenitors. In essence focus has been shifted away from the CLP to the MLP{{Citation needed|date=August 2010}} (lymphoid-specified progenitors), which are clearly lymphoid progenitors yet retain some myeloid potential, particularly the interesting ability in both man and mouse to make macrophages - one of the most versatile of immune cell defenders - and also many dendritic cells, the best 'watchdogs' of antigen invaders.

===In brief===
However, whatever the details may turn out to be, the process of lymphopoiesis always seems to relentlessly give rise to progeny with special attributes and abilities - 'superpowers' so to speak - but with progressively more restricted lymphoid developmental potential.

==Stages of development==
===The old model: Lymphoid vs Myeloid===
This model of lymphopoiesis had the virtue of relative simplicity, agreement with nomenclature and terminology, and is essentially valid for the favorite lab animal, the mouse.
# '''pHSC''' pluripotent, self-renewing, hematopoietic stem cells <ref>''Medical Immunology'', Litwin, p.115</ref> which give rise to
# '''MPP'''   multipotent progenitors, which give rise to
# '''ELP'''   (or PRO) [[Prolymphocytes]], early lymphoid progenitors, and finally to the
# '''CLP'''   Common lymphoid progenitor, a cell type fully committed to the lymphoid lineage.

pHSC, MPP and ELP cells are not fully committed to the lymphoid lineage because if one is removed to a different location it may differentiate into non-lymphoid progeny. However CLP ''are'' committed to the lymphoid lineage. The CLP is the transit cell responsible for these (generally parallel) stages of development, below: 

*NK cells   
*Dendritic cells (lymphoid lineage; DC2 <ref>''Textbook of Medical Immunology'', page 31</ref>)
*Progenitor B cells
**Pro-B cells => Early Pro (or pre-pre)-B cells => Late Pro (or pre-pre)-B cells
**Large Pre-B cells => Small Pre-B cells
**Immature B cells
**B Cells => (B1 cells; B2 cells)
***Plasma cells
* Pro-T cells   
** T-cells

=== New model: Mixed Myeloid/Lymphoid model===
====Research on new models====
*The Common Myelolymphoid Progenitor: A Key Intermediate Stage in Hemopoiesis Generating T and B Cells <ref>The Common Myelolymphoid Progenitor: A Key Intermediate Stage in Hemopoiesis Generating T and B Cells; Min Lu, Hiroshi Kawamoto, Yoshihiro Katsube, Tomokatsu Ikawa, and Yoshimoto Katsura; ''J. Immunol.'' 2002;169;3519-3525</ref>
*Identification of Flt3 + Lympho-Myeloid Stem Cells Lacking Erythro-Megakaryocytic Potential: A Revised Road Map for Adult Blood Lineage Commitment <ref>Identification of Flt3 + Lympho-Myeloid Stem Cells Lacking Erythro-Megakaryocytic Potential: A Revised Road Map for Adult Blood Lineage Commitment; Lund Strategic Research Center for Stem Cell Biology;  Cell; Vol. 121, 295–306, April 22, 2005</ref>
*Adult T-cell progenitors retain myeloid potential <ref>Adult T-cell progenitors retain myeloid potential; Haruka Wada, Kyoko Masuda, Rumi Satoh , Kiyokazu Kakugawa, Tomokatsu Ikawa, Yoshimoto Katsura & Hiroshi Kawamoto; ''Nature'' Vol 452 10 April 2008</ref>
*Research Findings May Shed Light on T-cell Leukemias and Immunodeficiencies <ref>[http://www.penncancer.org/penn_news.cfm?ID=449 Research Findings May Shed Light on T-cell Leukemias and Immunodeficiencies] April 9, 2008;</ref>
*Blood Lines Redrawn <ref>Blood Lines Redrawn; Thomas Graf; ''Nature'' Vol 452 10 April 2008  p.702-703</ref>
*The earliest thymic progenitors for T cells possess myeloid lineage potential <ref>The earliest thymic progenitors for T cells possess myeloid lineage potential; J. Jeremiah Bell, Avinash Bhandoola; ''Nature''; Vol 452, 10 April 2008, p. 764-768</ref>
*Revised map of the human progenitor hierarchy shows the origin of macrophages and dendritic  cells in early lymphoid development <ref>Revised map of the human progenitor hierarchy shows the origin of macrophages and dendritic  cells in early lymphoid development; Dick et al; ''Nature Immunology'';  Volume 11  Number 7 July 2010  p. 585-595</ref>
*Not a split decision for human hematopoiesis <ref>Not a split decision for human hematopoiesis; Kenneth Dorshkind; ''Nature Immunology'' Volume 11   Number 7 July 2010  p. 569-570</ref>

==Graphical view of the old model vs Mixed Myelo-Lymphoid model==
<gallery>
Image:New_Mixed_Myeloid-Lymphoid_Progenitor_Tree(RCCH)_Grayscale.jpg | Side by side. Comparing the new and old lineage models.
Image:MLP to DN3 resized annotated-Aug 3 2010.jpg | Revised Lineage Myelo-lymphoid flowchart.
</gallery>

==General immunology reference texts==

'''Texts in bold''' are the most heavily cited in this article.

*''Cell Communication in Nervous and Immune System''; Gundelfinger, Seidenbecher, Schraven; Springer Berlin Heidelberg New York; 2006; ISBN 3-540-36828-0 
*''Color Atlas of Hematology''; Theml et al; Thieme; 2004; ISBN 1-58890-193-9
*''Dynamics of Cancer''; Steven A. Frank; Princeton University Press, Princeton, New Jersey; 2007; ISBN 0–691–13365–2, Creative Commons Public License
*''Fundamental Immunology'', 5th edition; William E. Paul (Editor); Lippincott Williams & Wilkins Publishers; 2003; ISBN 0-7817-3514-9
*''Immuno-Biology: The Immune System in Health and Science'', 6th Edition; Janeway, Travers; 2005; Garland Science Publishing, New York; ISBN 0-8153-4101-6
*''Immunology Introductory Textbook'' (ebook;revised 2nd edition); Nandini Shetty; New Age International (P) Limited, Publishers, India; 2005; ISBN  81-224-2335-3
*'''''Instant Notes in Immunology''''', 2nd ed.; Lydyard, Whelan, Fanger; Taylor and Francis Group; 2004; China Version ISBN 703-025225-8； 46RMB Wangfujing Bookstore
*'''''Medical Immunology'''''—6th ed.; G. Virella, Editor; Informa Healthcare USA, Inc; 2007; ISBN 0-8493-9696-0
*''Stem Cell Biology''; Marshak, Gardner, Gottlieb; Cold Spring Harbor Laboratory Press; 2001; ISBN 0-87969-575-7/01
*'''''Textbook of Human Development and Histology'''''; Zhong Cuiping et al; Shanghai Scientific and Technical Publishers; 2006; ISBN 7-5323-8230-3
*'''''Textbook of Medical Immunology''''' (Immunology, 7th Edition); LIM Pak Leong; Elsevier (Singapore) Pte Ltd.; 2006; ISBN 0-323-03397-7 

==References==
{{Reflist}}

==Additional images==
Alternate views of lineages
<gallery>
Image:Illu blood cell lineage.jpg|Blood cell lineage. For scale, note that megakaryocytes (50-100 μm) are 10 to 15 times larger than a typical red blood cell.
Image:Ch12f3_from_NIH.jpg|Blood cell lineage. Based on self-renewal ability.
Image:Hematopoiesis_(human)_diagram.png |Schematic view. Well-defined lineages.
Image:New_Mixed_Myeloid-Lymphoid_Progenitor_Tree(RCCH)_Grayscale.jpg | Side by side. Comparing the new and old lineage models.
</gallery>

==External links==
* [http://www.copewithcytokines.de/cope.cgi?key=Lymphopoiesis  The www.copewithcytokines.de Mini-portal to Lymphopoiesis terminology]
* {{eMedicineDictionary|lymphopoiesis}}
* {{MeshName|Lymphopoiesis}}
* {{DorlandsDict|five/000062114|Lymphopoiesis}} 
* [http://www.hematologica.pl/Atlas3/Angielska/SpisUC.htm Overview at hematologica.pl]

{{Blood}}

{{Lymphocytes}}
{{Lymphatic organ anatomy}}
{{Immune system}}

[[Category:Lymphatic system]]
[[Category:Lymphocytes]]
[[Category:Immunology]]
[[Category:Histology]]

[[fr:Leucopoïèse]]
[[Category:2Fix]]