B cell Biology
B cell development
All B cell lymphocytes are distinguished by having CD19+ on their surface. As they mature, they also display CD20+ and their function is to produce antibodies (immunoglobulins) which are either bound to their cell membrane (the B cell receptor - BCR) and presents foreign antigens to T cells (APC - antigen presenting cell function) or transform into plasma cells which secrete antibodies.
B cells are produced by myeloid cells in the bone marrow and go through several stages to maturity. During this period they develop their BCR through a similar process to the T cell receptor by a process of gene recombination, which from a large number of potential segments, randomly rearranges and recombines them into a final sequence to create a complete BCR, replicated many times on its surface. The BCR may recogise self antigens and in a similar way that T cells with a TCR capable of autorecognition, they are eliminated (negative selection) if they bind strongly or not at all, otherwise they are positively selected.
As the B cell matures it display CD20+ on its surface and emerges from the bone marrow, enters the secondary lymphoid organs (spleen, lymph nodules, Peyer’s patches, tonsils, and mucosal tissue) to continue its differentiation.
After the naiive B cell is activated (see below) it may differentiate into a plasma cell to secrete antibodies (losing its CD20 marker), a regulatory B cell (CD20 negative) or a memory B cell (CD20 positive).
B cell activation
When B cell BCR binds to an antigen, either soluble, presented by another APC or membrane bound (Signal 1) the antigen it is ingested by the B cell, phagocytosed, and then presented on the Class II HLA glycoprotein where it is able to bind to a T helper cell with a TCR that recognises the same antigen (Figure 1). This provides the Signal 2 leading to a fully active B cell. In addition the T cell is activated. B cells can be activated without T helper cell assistance, usually by microbes providing a swift, but less sophisticated response. The activated B cell may be short lived or mature into a memory B cell.
Alternatively the B cell is transformed into a plasma cell which secretes antibody.
Figure 1. Altaileopard, Public domain, via Wikimedia Commons
- Mechanisms of central tolerance for B cell. D Namazee. Nature Reviews Immunology 2017;17:281
- Memory B Cells and Long-lived Plasma Cells. Ionescu and Urschel. Transplantation 2019;103: 890–898