COMPLEMENT ACTIVITY IN SERUM

Complement Activity in Serum

Complement Activity in Serum

The complement system is composed of at least eleven proteins (ß-globulins) normally found in blood serum. These proteins were named numerically in the order in which they were discovered, C1 through C9, although C1 is now known to be comprised of three separate proteins (C1q, C1r, and C1s) linked by calcium.

When the entire complement system through C9 is activated, the end result is cell lysis, usually bacterial lysis in vivo or erythrocyte lysis in clinical tests. There are three pathways by which this is accomplished. The classical pathway can generally be divided into a fixation phase and a lytic phase. The fixation phase is initiated when Ag-Ab complexes trigger C1, C2, and C4 reactions in the presence of the divalent cations of calcium and magnesium. An enzyme product, C4b2a, cleaves and activates C3 to C3a and C3b, the latter of which binds to the cell membrane. C5 is activated in serum by the membrane bound C4b2a3b (C5 convertase), forms a complex with C6 and C7, and is subsequently stabilized by binding to the cell membrane. Activation of C8 and C9 components follows, and upon their fixation to the cell membrane, lysis occurs.

The immune system is a vital physiological component that affords animals protection from disease and is composed of innate and adaptive mechanisms that rely on cellular and dissolved components. The serum complement system is a series of dissolved proteins that protect against a variety of pathogens. The activity of complement in serum can be determined by its ability to lyse red blood cells in vitro. Here, we describe a modification of a standard complement hemolysis assay that makes an interesting and informative laboratory exercise suitable for a variety of courses including physiology.

In the late 19th century, Hans Ernst August Buchner found that blood serum contained a "factor" or "principle" capable of killing bacteria. In 1896, Jules Bordet, a young Belgian scientist in Paris at the Pasteur Institute, demonstrated that this principle had two components: one that maintained this effect after being heated, and one that lost this effect after being heated. The heat-stable component was responsible for the immunity against specific microorganisms, whereas the heat-sensitive (heat-labile) component was responsible for the non-specific antimicrobial activity conferred by all normal serum. This heat-labile component is what we now call "complement."

The term "complement" was introduced by Paul Ehrlich in the ...