The document discusses several key concepts in immunology:
1) Antigens are molecules that bind to antibodies or are presented by MHC molecules to T cells. Self antigens are tolerated while non-self antigens trigger an immune response.
2) Exogenous antigens enter the body from outside and are taken up by antigen presenting cells and presented to T cells to activate the immune response.
3) Endogenous antigens are generated inside cells from infection or metabolism and are presented on MHC class I to activate cytotoxic CD8 T cells to destroy infected cells.
