FIG. 1.

Simplified schematic representation of immune mechanisms that can act to protect animals against invading viral, bacterial, and protozoal pathogens or against multicellular helminth parasites. Viral, bacterial, or protozoal pathogens (red ovals) that infect non-antigen-presenting cells can be killed by cytotoxic T cells (CTL) that recognize pathogen-derived epitopes presented in conjunction with major histocompatibility complex (MHC) class I on infected cells or by antibody-dependent lysis or opsonization of infected cells expressing pathogen molecules. Extracellular pathogens, or intracellular pathogens on their way to infect other cells, can be attacked by specific circulating antibodies and either killed by lysis or agglutination or phagocytosed by macrophages and neutrophils. Both antibody and CTL induction requires help from pathogen-specific CD4 helper T cells that are activated after interaction with pathogen-derived epitopes presented in conjunction with MHC class II molecules on the surface of MHC class II⁺ antigen-presenting cells. If pathogens infect antigen-presenting cells, they can be killed directly by CD4 T cells as well as CD8 CTL through the induction of mediators such as gamma interferon (IFN-γ), reactive oxygen and nitrogen species, and indoleamine 2,3-dioxygenase (IDO). Toxins released by pathogens (red circles) can be neutralized by circulating antibodies, thereby decreasing clinical signs of infection. Multicellular helminth parasites generally do not reside within host cells and are too large to be phagocytosed; therefore, they usually require alternative immune killer mechanisms mediated by antibody-directed actions of mast cells and eosinophils. Essential secreted proteins and toxins derived from the worms (brown circles) may also be neutralized by antibodies and thereby interfere with parasite growth.