Microbes, such as bacteria and viruses, are phagocytized much better in the presence of C3b because there are C3b receptors on the surface of many phagocytes.
C5a and the C5,6,7 complex attract neutrophils. They migrate especially well toward C5a. C5a also enhances the adhesiveness of neutrophils to the endothelium.
C3a, C4a, and C5a cause degranulation of mast cells with release of mediators (e.g., histamine), leading to increased vascular permeability and smooth muscle contraction, especially contraction of the bronchioles, leading to bronchospasm. Anaphylatoxins can also bind directly to smooth muscle cells of the bronchioles and cause bronchospasm. C5a is, by far, the most potent of these anaphylatoxins. Anaphylaxis caused by these complement components is less common than anaphylaxis caused by type I (IgE-mediated) hypersensitivity (see Chapter 65).
Insertion of the C5b,6,7,8,9 MAC into the cell membrane forms a “pore” in the membrane. This opening in the membrane results in the killing (lysis) of many types of cells, including erythrocytes, bacteria, and tumor cells. Gram-negative bacteria, especially Neisseria species, are very susceptible to the MAC. Cytolysis is not an enzymatic process; rather, it appears that insertion of the complex results in disruption of the membrane and the entry of water and electrolytes into the cell.
Enhancement of Antibody Production
The binding of C3b derivatives to its receptors on the surface of activated B cells (complement receptor 2 [CR2]) provides the signal 2, which greatly enhances antibody production compared with that by B cells that are activated by antigen alone (see Chapter 61). The clinical importance of this is that patients who are deficient in C3b produce significantly less antibody than do those with normal amounts of C3b. The low concentration of both antibody and C3b significantly impairs host defenses, resulting in multiple, severe pyogenic infections.