Activators of the classical pathway are primarily a____n-a____y complexes or
aggregated i_____s. Activators of the classical pathway include the IgG
subclasses IgG_, IgG_, and IgG_, but the most effective activator is the large
pentamer Ig_. Nonimmunologic activators of this pathway include DNA, C-
reactive p___n, certain c____r membranes, and t____n-like enzymes.
Activation of the classical pathway begins with the interaction of C_ with an
a____n-a____y complex. The C1 component is comprised of three distinct protein
molecules: C1_, C1_, and C1. The binding of the C1q component to the Fc
portion of the IgG or IgM molecule initiates the p____y. Changes in C1q causes
C1_ to enzymatically activate C1_.
In the classical pathway, activated C1s cleaves C4 into two fragments: C4a which
is released into the fl___d phase as an a____n and C4b which may bind directly to
the a____ing surface. C4b may also be released into the fluid phase as an
o____n. Activated C1 also is capable of cl___ing and a___ing C2 generating C2_
and C2_. A site on the C2a fragment allows it to bind to the surface-bound C4b to
form the complex C4b__.
In the classical pathway, the C4b2a complex is known as C3 c____e and is
capable of cleaving and activating C_. C3a is the smaller of the two fragments
produced and is released into the fluid phase as an a_____n. A (small) (large)
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 c_____e
and is capable of cleaving and activating C5. This is the first step to the formation
of the m___e a___k c_____x.
The primary activators of the alternative pathway are usually non-i_____l in
nature. They include bacterial l_____s, erythrocytes of certain species, v___s,
f_____i, and p____s. Aggregated IgE, IgA, and IgG subclass 4 are i_____l
activators of this pathway.
An initial requirement for activation of the alternative pathway is the presence of
C3_, which is c____ly generated in (small) (large) amounts by natural hydrolysis of
C3. Continuation of the alternative pathway occurs only if an activating s____e is
present to provide a b____g site for the C3b and p____t it from control protein