B cells play a critical role in adaptive immunity by producing antibodies, which are proteins that can recognize and bind to specific antigens. B cells do this by using their B cell receptors (BCRs), which are similar in structure to TCRs but are membrane-bound immunoglobulin molecules.
Like TCRs, BCRs are highly specific for particular antigens. The variable region of the BCR, which is responsible for antigen recognition, is formed by the combination of three gene segments: V, D, and J segments, which undergo somatic recombination to generate a diverse repertoire of BCRs.
When a B cell encounters an antigen that it recognizes, the antigen binds to the BCR, triggering a signaling cascade that leads to B cell activation and proliferation. Activated B cells undergo clonal expansion, producing many copies of themselves that can produce antibodies.
The antibodies produced by B cells are secreted forms of the BCRs and have the same antigen specificity as the BCR. Antibodies are Y-shaped molecules that consist of two identical heavy chains and two identical light chains, each with a variable region that can bind to a specific antigen. The variable regions of the heavy and light chains together form the antigen-binding site of the antibody.
Antibodies can recognize and bind to a wide range of antigens, including proteins, carbohydrates, lipids, and nucleic acids. The binding of antibodies to antigens can lead to the neutralization of the pathogen, activation of complement, and opsonization, which promotes phagocytosis by other immune cells.
Like T cells, B cells also have the ability to undergo somatic hypermutation, a process that increases the affinity of the antibody for its target antigen. This process allows the immune system to generate a more effective antibody response over time.
Vaccination is a strategy that takes advantage of the specificity of BCRs and antibodies by exposing individuals to harmless versions of a pathogen or its components, which elicits an immune response and provides protection against the pathogen in the future.