Transitional B cells are

B cells B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...

at an intermediate stage in their development between bone marrow immature cells and mature B cells in the

spleen The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .

. Primary B cell development takes place in the

bone marrow Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoieti ...

, where immature

B cell B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...

s must generate a functional

B cell receptor The B cell receptor (BCR) is a transmembrane protein on the surface of a B cell. A B cell receptor is composed of a membrane-bound immunoglobulin molecule and a signal transduction moiety. The former forms a type 1 transmembrane receptor protein, ...

(BCR) and overcome negative selection induced by reactivity with

autoantigens In immunology, autoimmunity is the system of immune responses of an organism against its own healthy cells, tissues and other normal body constituents. Any disease resulting from this type of immune response is termed an "autoimmune disease". ...

. Transitional cells can be found in the bone marrow,

peripheral blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in th ...

, and spleen, and only a fraction of the immature B cells that survive after the transitional stage become mature B cells in

secondary lymphoid organs The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system, and complementary to the circulatory system. It consists of a large network of lymphatic vessels, lymph nodes, lymphatic or lymphoid ...

such as the spleen.

Characteristic of transitional cells

The term "transitional B cell" was first used in

1995 File:1995 Events Collage V2.png, From left, clockwise: O.J. Simpson is acquitted of the murders of Nicole Brown Simpson and Ronald Goldman from the year prior in "The Trial of the Century" in the United States; The Great Hanshin earthquake strike ...

for cells that are developmentally intermediate between immature

B lineage cells and fully mature naïve B cells in the

and secondary lymphoid tissues, found in mice. In humans, it is postulated that the transitional cells, after leaving the bone marrow, are subjected to peripheral checks to prevent the production of

autoantibodies An autoantibody is an antibody (a type of protein) produced by the immune system that is directed against one or more of the individual's own proteins. Many autoimmune diseases (notably lupus erythematosus) are associated with such antibodies. P ...

. Transitional B cells that survive selection against autoreactivity develop eventually into naive B cells. Given the fact that only a small fraction of immature B cells survive the transition to the mature naive stage, the transitional B cell compartment is widely believed to represent a key negative selection checkpoint for autoreactive B cells. All transitional B cells are high in heat-stable antigen (HSA, CD24) relative to their mature counterparts and express the phenotypic surface markers AA4.

T1 and T2

There are two transitional stages for the B cells in mouse, T1 and T2, with the T1 stage occurring from its migration from the bone marrow to its entry into the spleen, and the T2 stage occurring within the spleen where they developed into mature B cells. As in the mouse, human transitional cells can be found in the bone marrow, peripheral blood, and spleen. However, in contrast to the nuanced models proposed in the mouse, thus far human studies have, by and large, described a rather homogeneous population of transitional B cells (T1/T2) defined by the expression of high levels of

CD24 Signal transducer CD24 also known as cluster of differentiation 24 or heat stable antigen CD24 (HSA) is a protein that in humans is encoded by the ''CD24'' gene. CD24 is a cell adhesion molecule. Function CD24 is a sialoglycoprotein expressed ...

CD38 CD38 (cluster of differentiation 38), also known as cyclic ADP ribose hydrolase is a glycoprotein found on the surface of many immune cells (white blood cells), including CD4+, CD8+, B lymphocytes and natural killer cells. CD38 also functions in ...

and

CD10 Neprilysin (), also known as membrane metallo-endopeptidase (MME), neutral endopeptidase (NEP), cluster of differentiation 10 (CD10), and common acute lymphoblastic leukemia antigen (CALLA) is an enzyme that in humans is encoded by the ''MME'' ge ...

. Overall there is general agreement on the markers used to separate the subpopulations, although some differences exist in the number of subgroups and in the functional characteristics of the T2 population. T1 B cells are distinguished from the other subsets by the following surface marker characteristics: they are

IgM Immunoglobulin M (IgM) is one of several isotypes of antibody (also known as immunoglobulin) that are produced by vertebrates. IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antige ...

^hi

IgD Immunoglobulin D (IgD) is an antibody Isotype (immunology), isotype that makes up about 1% of proteins in the plasma membranes of immature B-lymphocytes where it is usually co-expressed with another cell surface antibody called IgM. IgD is also pro ...

⁻

CD21 Complement receptor type 2 (CR2), also known as complement C3d receptor, Epstein-Barr virus receptor, and CD21 (cluster of differentiation 21), is a protein that in humans is encoded by the CR2 gene. CR2 is involved in the complement system. It ...

⁻

CD23 CD23, also known as Fc epsilon RII, or FcεRII, is the "low-affinity" receptor for IgE, an antibody isotype involved in allergy and resistance to parasites, and is important in regulation of IgE levels. Unlike many of the antibody receptors, CD ...

⁻, whereas T2 B cells retain high levels of surface IgM but are also IgD⁺CD21⁺ and CD23⁺. Differences in functional characteristics of the T2 subpopulation reported by different laboratories are unexplained, although they might be due to differences in isolation strategies. In any case, there is consensus that T2 B cells clearly differ functionally from T1 B cells.

References

{{Lymphocytes B cells Lymphocytes Bone marrow Human cells Immunology Immune system

Characteristic of transitional cells

T1 and T2

See also

References