In immunology, as with any science, there are moments of grand synthesis, when seemingly disparate observations and seemingly inexplicable phenomenon converge on a single mechanistic answer. How does the self know itself? Because every cell in your body expresses a set of histocompatibility (H2) proteins that are different from the proteins expressed by a stranger’s cells. When a stranger’s skin, or bone marrow, is implanted into your body, your T cells recognize these MHC proteins as foreign—nonself—and reject the invading cells.
