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