Complement System, C3 Deficiency

Complements are a group of blood serum proteins that are critically important in the defense against infection. The complement system includes at least 30 proteins. The first nine of these proteins were given numbers as their names, such as C1, C2, C3, etc. As more proteins were discovered, they were named with letters, such as Factor B and Factor D. Still, others were given more descriptive names such as C1 Inhibitor. These proteins act together to provide critical help in the defense against infection in a number of ways. One of the proteins, C3, acts to coat bacteria so that the bacteria are more easily ingested by white blood cells. Others, C7, C8 and C9, assemble on the surface of a certain kind of bacteria and punch holes in their walls, causing them to rupture and die. Finally, small fragments of two of the complement proteins, C3 and C5, can cause an increase in blood supply and attract white blood cells to areas of infection, both of which are needed to clear an infection.

Deficiencies in one or several complements may cause serious problems, but at this time, it is not possible to replace the missing components of the complement system. In general, complements have rapid turnover and often must be made by the body on a daily basis. Therefore, long-term replacement therapy is not an option since injections of highly purified components would be required almost every day and the proteins are difficult to purify. Patients with abnormalities that are associated with a high frequency of infection are usually helped by immunization when available and, occasionally, are treated with prophylactic antibiotics. Most patients with complement deficiencies can expect to become productive adults, if they are recognized as having the deficiency and treated early and vigorously.


  1. IDF Patient & Family Handbook For Primary Immunodeficiency Diseases, 4th Edition
  2. Johnson JP, McLean RH, Cork LC, Winkelstein JA. Genetic analysis of an inherited deficiency of the third component of complement in Brittany spaniel dogs. Am J Med Genet. 1986 Nov;25(3):557-62.
  3. Joanne R. Blum b, c, d, a, Linda C. Cork b, c, d, a, Jeanette M. Morris b, c, d, a, Jean L. Olson b, c, d, a and Jerry A. Winkelstein. The clinical manifestations of a genetically determined deficiency of the third component of complement in the dog.

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