The Questions of Developmental Biology

3. Antigens are presented (usually by macrophages) to the antibodies on the B cell membranes.
4. Only those B cells that bind to the antigen can complete their development into antibodysecreting
plasma cells. These B cells divide repeatedly, produce an extensive rough endoplasmic
reticulum, and synthesize enormous amounts of antibody molecules. These antibodies are
secreted into the blood.
5. The specificity of the antibody made by the plasma cell is exactly the same as that which was
on the cell surface of the B cells.
The type of antibody molecule on the cell surface of the B cell is determined by chance.
Out of the 10 million types of antibody proteins the cell can possibly synthesize, each B cell
makes only one type. That is, one B cell may be making antibodies that bind to poliovirus, while
a neighboring B cell might be making antibodies to diphtheria toxin. The B cells are continually
being created and destroyed. However, when an antigen binds to a set of B cells, these cells are
stimulated to divide and differentiate into plasma cells (that secrete the antibody) and memory
cells (that populate lymph nodes and respond rapidly when exposed to the same antigen later in
life) (Figure 21.16). Thus, each person's constellation of plasma cells and memory cells differs
depending on which antigens he or she has encountered. Identical twins have different
populations of B cell descendants in their spleens and lymph nodes.