The immune system has evolved to protect the host from a universe of pathogenic microbes
that are themselves constantly evolving. The immune system also helps the host eliminate
toxic or allergenic substances that enter through mucosal surfaces. Central to the immune
system's ability to mobilize a response to an invading pathogen, toxin, or allergen is its
ability to distinguish self from nonself. The host uses both innate and adaptive mechanisms
to detect and eliminate pathogenic microbes, and both of these mechanisms include self …