The immune system is able to distinguish between self and non-self through humoral and cellular defense mechanisms, respectively. B cell receptors and antibodies (Ab) bind 3-dimensional molecular structures of a large variety, among them proteins, lipids and carbohydrates, in their native or denatured form. T cells, on the other hand, are more limited. Via their T cell receptor (TCR), they recognize parts of processed pro-teins, peptides which have to be presented on special receptors, the

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... histocompatibility complex (MHC) molecules, on the cell surface. The 2 major subsets of T cells, CD8+ and CD4+ T cells, recognize 2 different types of MHC molecules, MHC class I and MHC class II. In the classical view, MHC class I molecules present peptides from endogenously synthesized proteins on the surface of the vast majority of cells, allowing circulating CD8+ cytotoxic T lymphocytes (CTL) to survey cells for possible infections or improper protein ex-