The extracellular/intracellular, or helper/killer T cell route of antigen presentation
MHC class I and II not only associate with antigens from different locations but they also differ in the kind of T cell to which they present antigens. External peptides presented in association with MHC class II are recognised by a T cell that has a surface receptor called CD4. CD4+ T cells are often referred to as helper T cells as their main job is to help other cells of the immune system to do their job well as seen here. For example, helping B cells to produce antibodies or helping another kind of T cells, the cytotoxic T cell, to do their job. These cytotoxic T cells are also called killer T cells as their main job is to kill, for example, infected self cells as seen here. Killer T cells have a different surface molecule, CD8, on their surface which makes it possible for them to interact with MHC class I. Hence, killer T cells become activated and kill when they recognise specific intracellular peptides presented with MHC class I.
This segregation of peptides from different locations, presented to different kinds of T cells, makes beautiful sense! In this way a cell filled with virus will be able to call for its own destruction by presenting viral peptides via MHC class I to killer T cells. Each and every cell in your body has therefore got MHC class I available for presentation of intracellular antigen. So you don't need to be an APC to activate the killer T cell! Shall we take a look at how processing of these internal antigens works which means that they get loaded on to MHC class I and not MHC class II? Follow me to the next page!