Selection of B cell clones and memory B cells.

During a humoral immune response, a variety of histological, cellular, and molecular changes occur within the immune system. In this review, we would like to summarize and integrate these changes with a view toward gaining insight into the process of clonal selection. We describe here how antigen receptor number limits the sensitivity of a B cell to transduce a signal in response to antigen. We have also seen that this limitation can be compensated for by expressing a higher affinity receptor for antigen. These data suggest that the down regulation of antigen binding receptors in germinal center cells might be designed to exploit anticipated increases in affinity that result from somatic hypermutation in these tissues. This hypothesis is consistent with observed biological changes that occur during an immune response and has bearing on both the mechanism of affinity maturation and generation of immunologic memory.