c. Activated C1s cleaves C4 into two fragments: C4a which is released into the
fluid phase as an anaphylatoxin and C4b which may bind directly to the activating
surface (Figure 2-5). C4b may also be released into the fluid phase as an opsonin.
Activated C1s is also capable of cleaving and activating C2, generating C2a and C2b.
A site on the C2a fragment allows it to bind to the surface-bound C4b to form the
d. The C4b2a complex is known as C3 convertase and is capable of cleaving
and activating C3. C3a is the smaller of the two fragments produced and is released
into the fluid phase as an anaphylatoxin. The larger C3b fragment may either be
released into the fluid phase as an opsonin or attach to the activating surface at a site
distinct from the C4b2a and antibody. Only a small portion of the total number of C3b
molecules bind to the activating surface and interact with C4b2a. The resulting
C4b2a3b complex is known as C5 convertase and is capable of cleaving and activating
C5. This is the first step to the formation of the membrane attack complex.
Figure 2-5. Classical pathway.