The immune system has evolved complex processes to ensure tolerance to autoantigens (self antigens) while preserving the potential to mount effective humoral (mediated by antibodies secreted by B lymphocytes, or B cells) and cellular (mediated by T lymphocytes, or T cells) immune responses against invading pathogens. The establishment of an immune response to various foreign antigens involves a complex interplay of not only pathogen-related factors that influence infectivity but also host-related factors that help mount, sustain, and control the necessary adaptive immune responses against the pathogen. Indeed, when a pathogen invades its host, an innate and T helper (Th) cell–dominated adaptive immune response is induced, clearing the infection and, most importantly, establishing life-long immunity to the pathogen. (Note that Th cells are also known as CD4+ cells.)