How do B cells recognize antigens
Synonym: B cell
English: B cell, B lymphocyte
B lymphocytes belong to the cell group of lymphocytes. They play an important role in the human immune system. The "B" in B-lymphocyte stands for "Bursa fabricii", a bird organ in which the B-lymphocytes were first described. In English, the letter "B" is often derived from "bone marrow".
B lymphocytes are rounded cells, about 6 µm in diameter, whose large nucleus is only surrounded by a narrow cytoplasmic rim.
The B-lymphocytes form the basis for the specific humoral immune system. They form antibodies in response to an antigen stimulus and are therefore responsible for the adaptive immune response.
B-lymphocytes start their life in the bone marrow as so-called pre-B-cells. These cells do not yet have any immunoglobulins on their cell surface. As they mature further, the gene segments that are responsible for coding the immunoglobulins are rearranged. This leads to the development of membrane-bound immunoglobulins, the so-called B-cell receptors. The B-lymphocyte now shows a population with IgM and IgD - this also allows it to be differentiated from other lymphocytes. In addition, the membrane is equipped with surface markers (CD molecules), which can also be used to identify the cells. These include, for example, CD19, CD20, CD21 and CD40.
The resulting B lymphocytes have not yet come into contact with the antigen. You are "antigen naive" and ready for further tasks. First of all, they circulate in the blood or reside in the lymphatic organs, where they are normally inactive. Only when the appropriate antigen binds to the B-cell receptor does the activation process begin. The antigen is taken up by the B lymphocytes, broken down and, together with MHC class II molecules, re-expressed as a complex on the cell surface. This complex can be recognized by a T helper cell, which then produces cytokines that ultimately activate the B lymphocytes.
3.3 Proliferation and Differentiation
The activated B-lymphocytes migrate to the center of a primary lymph follicle and form what is known as a germinal center in the B-cell regions of the secondary lymphatic organs. The activated B cells transform into centroblasts there. There is then a strong proliferation (division) and differentiation of the centroblasts in the lymph follicles, whereby the gene segments which encode the subsequent antibody production are subject to hypermutation. This creates centrocytes that are dependent on survival signals from the T helper cells and the follicular dendritic cells.
The receptor is then tested for its affinity for the antigen. Only cells with the right receptors receive survival signals, all other cells perish as a result of apoptosis. The plasma cells and B memory cells emerge from the remaining centrocytes through further differentiation.
- Plasma cells ("B plasma cells") are no longer able to divide B lymphocytes, which have reached their most mature stage of differentiation and secrete antibodies.
- B memory cells ("B memory cells") "store" the antigen information so that a faster immune response is available when the antigen comes into contact for the second time.
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