Activation of the classical complement pathway starts with binding of an activating substance to the Cl molecule, which activates the Cl sub-units, resulting in cleavage of C4 to C4b releasing C4a in the process. The short-lived C4b molecule can bind covalently to membranes. This process is limited to the immediate vicinity of the Cl complex. Because of the short life of the C4b molecule much of the C4d is free and circulates in serum.
Fixed-embedded human lymph node