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12 B Cell Activation Signal 1: Antigen in B cell receptor Signal 1: Antigen in MHC II recognized by antigen receptor Immune System Lecture 18, Part 2 Winter 2014 Signal 2: Co stimulatory molecules from dendritic cell Fig. 43.18 Intracellular pathogens 17 • How recognize? • Constant region • Variable region • Who recognizes? – T cells – Cell-mediated immune response • How to activate cytotoxic T cells? 18 T Cells • ~ 10 million different T cells/body • Only binds to antigen fragments presented by host cells – MHC class I proteins – intracellular Cell-Mediated Response Signal 2: cytokines from Helper T cell Fig. 43.9 • Cytotoxic T cells • Helper T cells 19 20 T cell Activation • Dendritic cells presenting (viral) antigen in both MHC I and MHC II • Helper T cell activation Signal 1: CD8 T cell recognizes antigen in MHC I on dendritic cell Virus Signal 2: cytokines from Helper T cell Fig. 43.16 Fig. 43.17 1 21 Cell-Mediated Response 22 Clonal Selection of T cells • Perforin molecules • Granzymes • Cell fragments can be targeted by antibodies for disposal Fig. 43.20 Fig. 43.17 Primary & Secondary Immune Response 23 Variation in T cell and B cell Receptors 24 Constant region Variable region Fig. 43.15 Fig. 43.11 Variation in T cell and B cell Receptors 25 • How do we get the huge variability in receptors? • Variety of combinations Fig. 43.13 – Variable / Joining / Constant (light chain) • Recombinase • Recombination permanent for that B or T cell Fig. 43.9 26 Self vs. non-self • As lymphocytes mature, tested for self reactivity • T cells migrate to & mature in thymus • B cells mature in bone marrow • Destruction of self-reactive cells 2 27 Adaptive Immunity: Summary 28 Plasma cells (B cells) Adaptive immunity • What cellular changes need to occur? – Recognition of traits specific to particular pathogens • Vertebrates only • Mediated by lymphocytes – B cells & T cells • Specificity • Memory • Self/non-self recognition Fig. 43.21 29 Active & Passive Immunity 30 Immunoglobin (Ig) • Active Immunity – Formation of clones of memory cells • Passive Immunity – Transfer of antibodies from mother to child through the placenta or breast milk • Short lived (weeks to few months) • Immunization (vaccination) – Artificial inducement of active immunity – Introduction of antigens 31 • How can pathogens evade detection by the immune system? 3