key concept 41.4 The Humoral Adaptive Response Involves Antibodies

Every day in the human body, billions of B cells survive the test of clonal deletion and are released from the bone marrow into the circulatory system. B cells are the basis for the humoral immune response.

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  • Antibody proteins are immunoglobulins with four polypeptide chains—two identical light chains and two identical heavy chains, each with constant and variable regions.

  • IgG is the most common of the five classes of immunoglobulins.

  • DNA rearrangements and other mutations generate the diversity of immunoglobulins.

A B cell begins as an unexposed B cell with a receptor protein on its cell surface that is specific for a particular antigen. The cell is activated by antigen binding to this receptor, and after stimulation by a TH cell, it gives rise to a clone of plasma cells that make antibodies as well as to a smaller number of memory cells (see Figure 41.7). The stimulation occurs after the B cell presents antigen to a TH cell with a receptor that can recognize the antigen. The TH cell then secretes cytokines that stimulate the B cell to divide.

Activated plasma cells (effector B cells) can synthesize and secrete large amounts of antibody proteins—up to 2,000 molecules per second! All the plasma cells arising from a given B cell produce antibodies that are specific for the antigen that originally bound to the parent B cell. Thus antibody specificity is maintained as B cells proliferate.

Animation 41.2 Humoral Immune Response

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