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Immunology and Infection
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LECTURE #9 – CELL-MEDIATED IMMUNE RESPONSES After studying lecture #9, you should be familiar with the following concepts. 1. Cellular Immunity. Cell-mediated immune responses deal with intracellular microbes. Such microbes can arise by escaping the phagolysosome of phagocytic cells, or by directly entering cells, for example through interactions with cell surface receptors. Cell- mediated immune responses are mediated by T cells and always require the interaction of T cells with other cells. 2. T cell activation. Naïve T cells require two signals for their activation – detection of antigen through the TCR and delivery of second signals through antigen presenting cells (APCs). A classical second signal is provided by B7 on APCs. APCs only express B7 upon detection of microbial PAMPs. B7 binds CD28 on T cells and provides the second signal. In the absence of second signals, T cells become anergic. Activated T cells undergo a rapid expansion program known as clonal expansion, whereby the stimulated T cell undergoes multiple division cycles. Note that clonal expansion is strictly limited to activated T cells. “Bystander T cells” that do not express the appropriate TCRs do not expand. The expanded population of T cells then differentiates into effector T cells that immediately combat the infectious agent, or memory T cells that persist for prolonged periods after infection and respond rapidly to repeat exposure. 3. Antigen Recognition. Antigen recognition requires the
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