The complement system is made up of a series of about 25 proteins that work to "complement" the activity of antibodies in destroying bacteria, either by facilitating phagocytosis or by puncturing the bacterial cell membrane. Complement also helps to rid the body of antigen-antibody complexes. In carrying out these tasks, it induces an inflammatory response.

Complement proteins circulate in the blood in an inactive form. When the first of the complement substances is triggered-usually by antibody interlocked with an antigen-it sets in motion ripple effect. As each component is activated in turn, it acts upon the next in a precise sequence of carefully regulated steps known as the "complement cascade."

In the so-called "classical" pathway of complement activation, a series of proteins gives rise to a complex enzyme capable of cleaving a key protein, C3. In the "alternative" pathway-which can be triggered by suitable targets in the absence of antibody-C3 interacts with a different set of factors and enzymes. But both pathways end in creation of a unit known as the membrane attack complex. Inserted in the wall of the target cell, the membrane attack complex constitutes a channel that allows fluids and molecules to flow in and out. The target cell rapidly swells and bursts.

Meanwhile, various fragments flung off during the course of the cascade can produce other consequences. One byproduct causes mast cells and basophils to release their contents, producing the redness, warmth, and swelling of the inflammatory response. Another stimulates and attract neutrophils. Yet another, C3b, opsonizes or coats target cells so as to make them more palatable to phagocytes, which carry a special receptor for C3b.

The C3b fragment also appears to play a major role in the body's control of immune complexes. By opsonizing antigen-antibody complexes, C3b helps prevent the formation of large and insoluble (and thus potentially damaging) immune aggregates. Moreover, receptors for C3b are also present on red blood cells, which appear to use the receptors to pick up complement-coated immune complexes and deliver them to the Kupffer cells in the liver.