![]() The fifth section contrasts affinity and specificity. By contrast, in vivo inoculations with several different pathogens showed that the initial binding by natural antibodies lowered the concentrations of pathogens early in infection by one or two orders of magnitude. One in vitro study of HIV suggested that these background antibodies bind to the viruses with such low affinity that they do not interfere with infection. Natural antibodies from different B cell lineages form a diverse set that binds with low affinity to almost any antigen. Each natural antibody can bind with low affinity to many different epitopes. The fourth section describes "natural" antibodies, a class of naive IgM antibodies. This mutation and selection produces high-affinity antibodies, typically of type IgA or IgG. Mutant lineages that bind with higher affinity to the target antigen divide more rapidly and outcompete weaker-binding lineages. As the stimulated B cell clones divide rapidly, they also mutate their antibody-binding regions at a high rate. A particular epitope stimulates division of B cells with relatively higher-affinity IgM antibodies for the epitope. Naive B cells make IgM antibodies that typically bind with low affinity to epitopes. The third section introduces the different stages in the maturation of antibody specificity. A single paratope can bind to unrelated epitopes, and a single epitope can bind to unrelated paratopes. Paratopes and epitopes define complementary regions of shape and charge rather than particular amino acid compositions. Each paratope has about 15 amino acids, of which about 5 contribute most of the binding energy for epitopes. ![]() Antibodies have a variable region of about 50 amino acids that contains many overlapping paratopes. The second section focuses on the paratope, the part of the antibody molecule that binds to an epitope. A change in any of those 5 key amino acids can greatly reduce the strength of antibody binding. However, only about 5 of the parasite's amino acids contribute to the binding energy. ![]() An antibody bound to an epitope covers about 15 amino acids on the surface of a parasite molecule. The surfaces of parasite molecules contain many overlapping antibody-binding sites (epitopes). The first section discusses antibody recognition. The molecular determinants of specificity and cross-reactivity define the nature of antigenic variation and the selective processes that shape the distribution of variants in populations. Cross-reactivity measures the extent to which different antigens appear similar to the immune system. ![]() Specificity measures the degree to which the immune system differentiates between different antigens. Two terms frequently arise in discussions of recognition. In this chapter, I describe the attributes of host and parasite molecules that determine immune recognition. ![]()
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