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University of Alberta
Immunology and Infection

LECTURE #7 – ANTIGEN CAPTURE AND PRESENTATION After studying lecture #7, you should be familiar with the following concepts. 1. Antigen Presenting Cells (APCs). Professional APCs (e.g. dendritic cells, macrophages) capture microbial protein antigens from the skin, gastrointestinal tract, respiratory tract and the bloodstream. APCs process these antigens intro a diverse number of peptides and present them to T cells in lymph nodes. Presented antigens provide the first signal for T cell activation. APCs that have encountered microbial agents also express “costimulators” on their surface. These costimulators provide the second signal required for T cell activation. APCs also express cytokines that direct the T cell response. 2. The Major Histocompatability Complex (MHC). APCs present antigens on MHC class I or MHC class II molecules. Each mammal encodes multiple MHC I and MHC II molecules and each MHC allele is highly polymorphic. Thus, there is a huge array of MHC molecules in a given population. In addition, each MHC molecule can bind and present a large number of peptide antigens. Therefore, a single MHC is capable of presenting peptide antigens to a large number of different T cells. 3. Class I MHC Molecules. Class I MHC molecules are expressed on all nucleated cells. Each class I molecule consists of an α chain and a β2-microglobulin chain. The α1 and α2 domains bind the peptide antigen and account for the bulk of the polymorphism within the respective allele. The α3 domain is invariant among MHC class I molecules and binds CD8. This ensures that only CD8+ T cells interact with MHC I/antigen complexes. MHC class I molecules present antigens derived from cytoplasmic microbial proteins of infected cells. The proteins are cleaved into peptides in the cytosol and pumped into the ER by TAP, where they bind MHC class I molecules and form a stable MHC/peptide c
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