Cd8+ cells respond to mhc class i indicates viral infection (cytosolic protein) activation causes diffentiation into cytotoxic t cells (kill infected host cell) Cd4+ cells respond to mhc class ii indicates microbial infection (replicating intracellular vesicles) Activation causes differentiation into th1 or th2 cells. T cell costimulation signal 1 = antigen-mhc signal 2 = antigen independent costimulatory signal (cd28 on t cell an b7 on apc) **ensures that only apc"s can cause t-cell differentiation** Dc"s most important activator of t-cells; presents viral antigens (good for innate immunity) Macrophages present bacterial antigens (scavengers) (b7 inducible by microbial products) B cells (b7 inducible by microbial products (lps) (good for low level circulating ag) bind through. Ab (raymond removed a table describing antigen uptake, mhc expression, costimulation, antigen presentation, and location characteristics of b cells, macrophages, and dendritic cells. ) Note: differentiated, effector t cells stimulated by signal one alone! 1st stage: t cell encounters antigen/mhc complex on apc upregulate il-2 and il2r.