The human complement (C) system is composed of about 20 components in blood plasma (Chapter 1). Of these, many become deposited at sites of complement attack and are detected by immunohistochemical methods. C3b, C4b, and their fragments remain covalently bound on targets and components of the membrane attack complex (MAC) can become inserted into cell membranes. In addition, many components (like C1q and properdin) are stable enough and have such a high affinity for targets that they can remain in tissues for prolonged periods.