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How do B cells begin producing antibodies after encountering an antigen?

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Diagram of an antibody molecule showing two identical light chains and two identical heavy chains joined by disulfide bonds in a Y-shaped structure, with labeled regions for antigen binding and effector function. — antibody The four-chain structure of an antibody, or immunoglobulin, molecule. The basic unit is composed of two identical light (L) chains and two identical heavy (H) chains, which are held together by disulfide bonds to form a flexible Y shape. Each chain is composed of a variable (V) region and a constant (C) region.

B cells begin producing antibodies through a process of activation and differentiation triggered by the recognition of a specific antigen.12

Activation Process

A B cell is activated when its surface antigen receptors—which are membrane-anchored antibodies—bind to a complementary antigenic determinant (epitope). While some antigens can stimulate B cells directly, most require additional help from an activated helper T cell. This T-dependent activation involves:32
* Cell-to-cell interaction: A protein on the B cell surface (CD40) binds to a ligand on the activated helper T cell.3
* Cytokine stimulation: The helper T cell secretes cytokines that provide the necessary signals for the B cell to proceed.43

Clonal Expansion and Differentiation

Once activated, the B cell undergoes clonal expansion, multiplying into a group of identical cells. These cells then differentiate into plasma cells, which act as specialized factories that secrete large quantities of antibodies into the blood and lymph. These antibodies are modeled after the original receptors and work to neutralize or destroy the invading antigen. Some activated B cells instead become memory B cells, providing long-term immunity by enabling a faster response upon future exposure to the same antigen.1235