Cell-Mediated Defense against Infection

e principal function of the mammalian immune system is to combat infectious diseases. Immune responses to antigens and microbial pathogens are divided into those mediated by cells (i.e., cell-mediated immunity) and those mediated by antibodies (i.e., humoral immunity). e T lymphocyte, the “central player” in cell-mediated immunity, provides antigen speci€city and orchestrates the antimicrobial activities of cells that generally lack antigen speci€city. “Supporting players” in cell-mediated immune responses include dendritic cells (DCs), which present antigens to T cells and provide contextual information, and monocytes and macrophages, which carry out instructions provided by activated T cells. Antigen speci€city is conferred by T cells, which bear T-cell receptors (TCRs) responsible for the detection of pathogen-derived peptides presented on major histocompatibility complex (MHC) molecules. Primary encounter of naïve T cells with a pathogen results in their activation, proliferation, and di‚erentiation into e‚ector T cells. Aƒer pathogen clearance, e‚ector T cells contract and give rise to long-term memory T-cell populations. Our understanding of T-cell responses to microbial pathogens and of the complexity of cell-mediated immune responses have increased dramatically. e discussion in this chapter is focused on our current knowledge regarding T-cell responses to microbial pathogens.