<unk>  stimulates T cells to become either "cytotoxic" CD8+ cells or "helper" CD4+ cells.<unk>  is a vital immune process that is essential for T cell immune response triggering. Because T cells recognize only fragmented<unk> s displayed on cell surfaces,<unk>  processing must occur before the<unk>  fragment, now bound to the major histocompatibility complex (MHC), is transported to the surface of the cell, a process known as<unk> , where it can be recognized by a T-cell receptor. If there has been an infection with viruses or bacteria, the cell will present an endogenous or exogenous peptide fragment derived from the<unk>  by MHC molecules. There are two types of MHC molecules which differ in the behaviour of the<unk> s: MHC class I molecules (MHC-I) bind peptides from the cell cytosol, while peptides generated in the endocytic vesicles after internalisation are bound to MHC class II (MHC-II). Cellular membranes separate these two cellular environments - intracellular and extracellular. Each T cell can only recognize tens to hundreds of copies of a unique sequence of a single peptide among thousands of other peptides presented on the same cell, because an MHC molecule in one cell can bind to quite a large range of peptides. Predicting which (fragments of)<unk> s will be presented to the immune system by a certain MHC/HLA type is difficult, but the technology involved is improving.
Antigen presentation
Drug metabolism
Alloantigen recognition
Drug recycling
Oocyte selection
Excretion
Reuptake
Immune response