The molecular mimicry shown by pathogens turns out to be at least as subtle, complicated, and full of surprises as the visual mim- icry shown by butterflies and other animals. Deceptive resemblances to human proteins are shown by the surfaces of various parasitic worms, protozoa, and bacteria. If there is any deficiency in the mim- icry of human tissues by a bacterium, we can expect it to evolve an improvement rapidly. Pathogen surfaces may have a complex sculp- turing of convexities and concavities, and the molecular forms most readily recognized by antibodies are hidden in crevices. As noted in the last chapter, some pathogens alter their exposed molecular struc- tures so rapidly that the host has difficulty producing newly needed antibodies fast enough. This is rapid change without evolution, because the same pathogen genotype codes for a variety of molecular structures.
Mimicry may not only permit pathogens to escape from immuno- logical attack but also make active use of hosts’ cellular processes. For instance, streptococcal bacteria make molecules similar to host hormones that have receptor sites on cell membranes. In effect, the bacterium has a key to the lock on the door that normally admits a hormone. Once inside the cell, the bacterium is shielded from immunological and other host defenses. The host has an endosome- lysosome complex that can attack pathogens within its cells, but molecular mimicry and other countermeasures protect the pathogen there too.