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One important difference between BCRs and TCRs is the way they can interact with antigenic epitopes. Whereas TCRs can only interact with antigenic epitopes that are presented within the antigen-binding cleft of MHC I or MHC II , BCRs do not require antigen presentation with MHC; they can interact with epitopes on free antigens or with epitopes displayed on the surface of intact pathogens. Another important difference is that TCRs only recognize protein epitopes, whereas BCRs can recognize epitopes associated with different molecular classes (e.g., proteins, polysaccharides, lipopolysaccharides).

Activation of B cells occurs through different mechanisms depending on the molecular class of the antigen. Activation of a B cell by a protein antigen requires the B cell to function as an APC, presenting the protein epitopes with MHC II to helper T cells. Because of their dependence on T cells for activation of B cells , protein antigens are classified as T-dependent antigens . In contrast, polysaccharides, lipopolysaccharides, and other nonprotein antigens are considered T-independent antigens because they can activate B cells without antigen processing and presentation to T cells.

A B cell plasma membrane has two long rectangles spanning it; these form a Y shape. Two shorter rectangles sit on the outside of the upper portion of the Y. The region spanning the membrane and half-way through the bars of the Y is the constant region. The upper region is the variable region which has the antigen binding sites. The long rectangles are the heavy chain. The shorter rectangles are the light chains. Multiple disulfide bridges hold the constant region together.
B-cell receptors are embedded in the membranes of B cells. The variable regions of all of the receptors on a single cell bind the same specific antigen.
  • What types of molecules serve as the BCR?
  • What are the differences between TCRs and BCRs with respect to antigen recognition?
  • Which molecule classes are T-dependent antigens and which are T-independent antigens?

T cell-independent activation of b cells

Activation of B cells without the cooperation of helper T cells is referred to as T cell-independent activation and occurs when BCRs interact with T-independent antigens. T-independent antigens (e.g., polysaccharide capsules, lipopolysaccharide) have repetitive epitope units within their structure, and this repetition allows for the cross-linkage of multiple BCRs, providing the first signal for activation ( [link] ). Because T cells are not involved, the second signal has to come from other sources, such as interactions of toll-like receptors with PAMPs or interactions with factors from the complement system .

Once a B cell is activated, it undergoes clonal proliferation and daughter cells differentiate into plasma cells. Plasma cells are antibody factories that secrete large quantities of antibodies. After differentiation, the surface BCRs disappear and the plasma cell secretes pentameric IgM molecules that have the same antigen specificity as the BCRs ( [link] ).

The T cell-independent response is short-lived and does not result in the production of memory B cells . Thus it will not result in a secondary response to subsequent exposures to T-independent antigens.

A circle with small chains of hexagons projecting from the surface is a pathogenic bacterial cell. The chains are polysaccharide antigens with repeating epitopes. Antibodies on the B cell bind to these epitopes. This causes the activation of the  B cell and secretion of pentameric IgM.
T-independent antigens have repeating epitopes that can induce B cell recognition and activation without involvement from T cells. A second signal, such as interaction of TLRs with PAMPs (not shown), is also required for activation of the B cell. Once activated, the B cell proliferates and differentiates into antibody-secreting plasma cells.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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