Neutrophil activation and chemotaxis.
Enhanced opsonization and phagocytosis.
Lysis of target cells, bacteria, and viruses.
b. Many of these effects are due to complement cleavage products known as
anaphylatoxins. You are probably already familiar with the related term anaphylaxis,
which is used to refer to an exaggerated allergic reaction.
THE CLASSICAL PATHWAY
a. Activators of the classical pathway are primarily antigen-antibody complexes
or aggregated immunoglobulins. Human immunoglobulins belonging to IgG1, IgG2, and
IgG3 subclasses and IgM are capable of initiating the classical pathway. The most
effective activator, however, is the large pentamer IgM. Nonimmunologic activators of
this pathway include DNA, C-reactive protein, certain cellular membranes, and
b. Activation of the classical pathway begins with the interaction of C1 with an
antigen-antibody complex. The C1 component is comprised of three distinct protein
molecules: C1q, C1r, and C1s. The binding of the C1q component to the Fc portion of
the IgG or IgM molecule initiates the pathway (Figure 2-4). Changes in C1q causes C1r
to enzymatically activate C1s.
Figure 2-4. C1 molecule.