B Lymphocytes
المؤلف:
APURBA S. SASTRY , SANDHYA BHAT
المصدر:
Essentials Of Medical Microbiology 2021
الجزء والصفحة:
3rd edition , p174-176
2025-08-28
467
B lymphocytes are the mediators of humoral immunity; constitutes 10–15% of blood lymphocytes. They are named after their site of maturation (bursa of Fabricius in birds and bone marrow in humans and other mammals). B cells proliferate through various stages, first in bone marrow, then in peripheral lymphoid organs (Fig. 1)
Fig1. B cell development.
Development of B Cells in Bone Marrow
Initial stages of B cell proliferation occur in bone marrow; independent of exposure to antigen.
- Pro-B Cells (Progenitor B Cells): They are the earliest bone marrow cells of B cell lineage. They do not produce immunoglobulins (Ig) but express a heterodimer Igα/Igβ that forms a part of the B cell receptor (BCR) in future
- Pre-B Cells (Precursor B Cells): Pro-B cells differentiate into pre-B cells by expressing μ heavy chain
- Immature B Cells: Pre-B cells proliferate into immature B cells by expressing light chain
* B cell receptor: Heavy chain μ and its light chain join to form complete IgM molecule. IgM is then complexed with heterodimer Igα/Igβ on the B cell surface to form B cell receptor (Fig. 2)
* Tolerance: Some of the immature B cells are capable of reacting to self-antigens. Tolerance to those B cells are essential for prevention of autoimmunity. Following contact with a self-antigen, the tolerance is developed either by:
^ Receptor editing: A process by which the Ig genes coding light chains are rearranged so that a different (edited) B cell receptor is produced which no longer reacts to self-antigen or
^ Negative selection: By apoptosis of self-reacting immature B cells in spleen.
Fig2. Structure of B cell receptor (BCR) and pre-BCR.
Development of B Cells in Peripheral Lymphoid Organs
Immature B cells migrate from bone marrow to peripheral lymphoid organs (lymph node and spleen) where they transform into mature B cells following contact with appropriate antigen.
Mature or Naive B Cells
Most mature B cells (95%) belong to the follicular B cell type and produce surface receptor IgD in addition to IgM.
They play an important role in humoral immune response
- Following antigenic stimulus, the mature B cells transform into activated B cells (lymphoblasts) which further differentiate into either effector B cells, i.e. plasma cells (majority) or memory B cells
- Plasma cells (antibody secreting cells): They are oval, large (15 µm size), with an eccentrically oval nucleus containing large blocks of peripheral chromatin (cartwheel appearance) and the cytoplasm containing abundant organelles. They have a short life span of two or three days.
However, there are few rare mature B cell types such as B-1 cell and marginal zone B cells which have limited diversity and are components of innate immunity.
- B-1 cells: They are found mostly in the peritoneal cavity, coated by surface markers IgM (natural antibodies) and CD5 molecules, but lack IgD
- Marginal-zone B cells: They are present at the edges of lymphoid follicles of spleen and are produced in response to the polysaccharide antigens.
B cells are the main components of humoral immunity; produce five classes of antibodies, which in turn have various biological functions
Differences between T cell and B cell are given in Table 1.
Table1. Differences between t cell and B cell.
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