Figure 3. Interactions between antigen-presenting cells (APCs) and T helper (CD4) cells required for T-cell activation.

The immunologic activation of most B cells requires a series of events (figure 4).

1. Clonal selection and binding of antigen.In this case, a precommitted B cell of a particular clonal specificity picks up the antigen on its Ig receptors and processes it into small peptide determinants. The antigen is then bound to the MHC II receptors on the B cell. The MHC/Ag receptor on the B cell is bound by a TH cell.

2. Instruction by chemical mediators.The B cell receives developmental signals from macrophages and T cells (interleukin-2 and interleukin-6) and various other growth factors, such as IL-4 and IL-5.

3.The combination of these stimuli on the membrane receptors causes a signal to be transmitted internally to the B-cell nucleus.

4.These events trigger B-cell activation. An activated B cell called a lymphoblast undergoes an increase in size and DNA and protein synthesis, in preparation for entering the cell cycle and mitosis.

5–6. Clonal expansion.A stimulated B cell multiplies through successive mitotic divisions and produces a large population of genetically identical daughter cells. Some cells that stop short of becoming fully differentiated are memory cells, which remain for long periods to react with that same antigen at a later time. This reaction also expands the clone size, so that subsequent exposure to that antigen provides more cells with that specificity. This expansion of the clone size is one factor in the increased memory response. The most numerous progeny are large, specialized, terminally differentiated B cells called plasma cells.

7. Antibody production and secretion.The primary action of plasma cells is to secrete into the surrounding tissues copious amounts of antibodies with the same specificity as the original receptor (figure 4). Although an individual plasma cell can produce around 2,000 antibodies per second, production does not continue indefinitely. The plasma cells do not survive for long and deteriorate after they have synthesized antibodies.