Which one of the following statements about humoral immune responses is true?
A. Naive B cells are required for initiation of primary responses and memory B cells are required for initiation of secondary responses.
B. Antibody responses to bacterial polysaccharide antigens require CD4+ helper T cells.
C. Heavy chain isotype switching typically occurs in response to bacterial polysaccharide antigens.
D. Affinity maturation does not require helper T cells.
E. Antibody-secreting cells generated during a humoral immune response live for only a few hours.
ANS: A. Humoral responses require antigen-dependent activation of B cells through binding of the antigen to membrane Ig on naive B cells or on memory B cells, for primary or secondary responses, respectively. In most cases, nonprotein antigens do not stimulate isotype switching or affinity maturation because these changes require T cell help, and only protein antigens can stimulate T cells. For both nonprotein and protein antigens, antibody-secreting cells that are generated may live for months, often in the bone marrow.
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