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Lecture 4

Lecture 4 Specific Immunity.doc

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Michelle French

Lecture 4 Specific Immunity: 1) SPECIFICITY • B and T cells bind and respond to foregin or abnormal molecules (antigens). • Antigens are typically complex protein or polysaccharide components of viruses, bacteria, fungi, protozoa, parasitic worms, pollen, transplanted tissue, and tumor cells. • Each antigen as a unique structure and contains different recognition sites called epitopes (antigen determinants), each of which can be detected by specific lymphocytes, which then target that invader for destruction. • A typically antibody is a Y-shaped molecule consisting of 4 protein chains: 2 identical heavy chains, 2 identical light changes (joined by disulfide bridges BasisofAntibody–Antig en specificity • Twoheavychains • Twolightchains • Constantregion— samewithinaclassof antibodies • Variableregion— differsfordifferent antigens,givesspecificity toantigen-bindingsite • Twoantigen-binding sites Figure23.5a BasisofAntibody–Antig en specificity Figure23.5b • Each contains variable regions (V) and constant regions (C). C consists of portions of heavy and light chains that make up the tail of the Y. • The V of an antibody have amino acid sequences that vary extensively from antibody to antibody, consist of portions of heavy and light chains that make up the top of the Y. 2 identical V regions form 2 identical antigen-binding sites (bind to two epitopes of the same kind). • An enzyme and its substrate show specificity. • Antigen receptors on the B cells are similar to antibody molecules except that the receptors are bound to the plasma membrane, whereas antibodies are secreted into the extracellular fluid. B cell antigen receptors are often called membrane antibodies (membrane immunoglobulins). A B cell’s membrane antibodies have the same specificity as the antibodies it later secretes as a plasma cell • T cells have T cell receptors (TCRs). Different in structure. Act as cell surface receptors for antigen. 2) DIVERSITY : • A single T lymphocyte or B lymphocyte has about 100 000 antigen receptors, all with the same specificity. • Antigen receptor molecules lymphocyte products are determined by random genetic events that occur early in the development of lymphocytes. • When a particular microorganism invades the body, it interacts with and activates only those lymphocytes that have receptors specific for the antigens its possesses. • Foreign antigen triggers an immune response against itself. This antigen-driven activation of lymphocytes is called clonal selection (necessary for immune responses). • Lymphocyte differentiation gives rise t
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