Complement or alexin (protective material), is a substance found in normal blood serum. It is called complement, abbreviated C’, because of its complementary effect on certain reactions between bacterial cells, bacteriolysins and other lysins. When associated with appropriate antibodies, it is detrimental to some bacteria, especially gram-negative bacteria, which it kills. Combinations of C’ and appropriate antibody lyse mammalian cells. Complement is not a true antibody, but it enhances the activity of true antibodies. In the absence of C’, cytolysis (cell breakdown) does not occur even when specific lysins are present to act on susceptible cells.

When complement reacts, in the presence of calcium, which is required, with the antigen-antibody complexes containing erythrocytes and hemolysin, the hemoglobin is released from the red blood cells resulting in hemolysis. Killing or phagocytosis of bacteria through the cooperative action of antibody and C’ is called a bactericidal effect. Complement is present in the blood serum of all normal animals but varies in amounts between species and individuals of the same species. Unlike antibody, C’ is not specific for a particular antigen and does not increase upon immunization. Complement consists of at least nine serum globulin components which react in a certain sequence to combine antigen and antibody. C’ is inactivated by heat, 56°C for 30 minutes. The complement-fixation reaction is a very useful test for the diagnosis of bacterial and viral infections. The best source of complement for serological tests in the laboratory is guinea pig serum. PROPERDIN Properdin is a high-molecular-weight serum protein found in normal mammalian blood. The name is derived from the Latin, pro- and perdere, meaning “to prepare to destroy”. It is a factor in serum thought to be associated with natural resistance to infection. Its action can be demonstrated in vitro, provided C’ factor of complement and magnesium ions are present. Properdin resembles the classical antibodies but has a broader spectrum of activity and less serological specificity. Experimentally induced changes in properdin levels of laboratory animals alter their susceptibility to bacterial infections. It is still too soon to evaluate the importance of the properdin system in immunity and infection. INTERFERON Interferon is an antiviral substance produced in animal tissues which inhibits virus growth and provides some protection to man and other animals against virus infections. It reacts against a wide variety or viruses. Although present in healthy individuals, it increases upon stimulation by virus infection. Apparent “natural” occurrence of interferon may be the result of previous stimulation by viral antigens. It can be administered to provide passive protection against virus infection. Interferon differs from antibodies in several ways. It is more useful in the treatment of established virus infections than in prophylaxis. It appears to act by preventing or slowing virus multiplication until the natural defenses of the body can be mobilized and take charge. It is produced inside the infected host cells early in the course of infection and alters them in such a way that they cannot support virus growth. Although there is a great deal to be learned about interferon, including its chemical, how it is produced, and how it acts, it is a very interesting substance which may be valuable in ways not suspected at present.