Complement component 3

Alternative names:

C3

What the risks are:

• excessive bleeding
• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin)
• infection (a slight risk any time the skin is broken)
• multiple punctures to locate veins

Special considerations:

Serum complement comprises a group of proteins that facilitate immunological and inflammatory responses. The so-called complement cascade involves a series of enzymatic reactions that take place in the blood. There are 9 major components labeled C1 through C9. The cascade can be initiated by various means, especially antigen-antibody complexes. The end-product of the cascade is the so-called membrane attack unit (also called terminal complement component), which creates holes in the membranes of attacking microorganisms, thereby causing lysis and death of the cells. CH50 or CH100 are 50% or 100% of 'whole' complement activity. There are also a number of side products of the complement cascade that attract white blood cells and increase the efficiency to engulf bacteria by phagocytic white blood cells.

When many microorganisms are exposed to fresh serum, complement is activated by an "alternative pathway" that does not require the presence of specific antibodies to the microorganism. These organisms are able to bind C3 directly. Bound C3, or some modified form of C3, is able to associate with factor D, factor B, and properdin to form a stable unit capable of continuing the complement cascade.

Typically, other tests that are more specific for the suspected disease are performed first.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.