Ch 6 Effector Mechanisms of T Cell-Mediated Immunity Flashcards
What are the types of T lymphocyte–mediated immune reactions that eliminate microbes that are sequestered in the vesicles of phagocytes and microbes that live in the cytoplasm of infected host cells?
Intracellular microbes that reside in phagosomes are eliminated by helper T cells, especially those of the TH1 subset that activate phagocytes to destroy ingested microbes. Microbes that reside in the cytoplasm may be eliminated by CD8+ T cell–mediated killing of the infected cells, thus eliminating the reservoir of infection.
Why do differentiated effector T cells (which have been activated by antigen) migrate preferentially to tissues that are sites of infection and not to lymph nodes?
Differentiated effector T cells lose expression of L-selectin and CCR7 (both of which are present on naive T cells) and can no longer home to lymph nodes. Effector cells bind to adhesion molecules on endothelium exposed to inflammatory cytokines and respond to chemokines produced at sites of inflammation, thus preferentially migrating to these sites.
What are the mechanisms by which T cells activate macrophages, and what are the responses of macrophages that result in the killing of ingested microbes?
Activated helper T cells secrete cytokines such as interferon-γ that activate macrophages. These helper T cells also express CD40 ligand, which can activate macrophages by engaging CD40. Activated macrophages then induce NADPH oxidase activity to generate reactive oxygen species, and nitric oxide synthase to make nitric oxide. These free radicals can destroy ingested microbes. Activated macrophages also produce increased amounts of lysosomal enzymes, which help to destroy microbes, and other molecules that promote inflammation and call in more leukocytes into the reaction.
What are the roles of TH1, TH17, and TH2 cells in defense against intracellular microbes and helminthic parasites?
TH1 cells eliminate intracellular pathogens by activating macrophages. TH2 cells can induce class switching to IgE and activate eosinophils to secrete proteins that kill helminths. TH2 cytokines enhance gut motility, which can help clear intestinal parasites.
How do CD8+ CTLs kill cells infected with viruses?
Activated CD8+ T cells secrete perforin and granzymes, which enter the infected cells recognized by the T lymphocytes and induce apoptosis of these infected cells.
What are some of the mechanisms by which intracellular microbes resist the effector mechanisms of cell-mediated immunity?
Some intracellular microbes evade immunity by preventing phagolysosomal fusion. Other intracellular microbes express molecules that can inactivate host complement responses. Some microbes are encapsulated and can resist phagocytosis and complement.
