At the heart of the immune response is the ability to distinguish between "self" and "non-self." Every cell in your body carries the same set of distinctive surface proteins that distinguish you as "self." Normally your immune cells do not attack your own body tissues, which all carry the same pattern of self-markers; rather, your immune system coexists peaceably with your other body cells in a state known as self-tolerance.

This set of unique markers on human cells is called the major histocompatibility complex (MHC). There are two classes: MHC Class I proteins, which are on all cells, and MHC Class II proteins, which are only on certain specialized cells.