There are two groups of protective proteins in the immune system that act non-specifically to protect the body. They are known as complement and interferons.
Complement is a name given to a number of complex, inactive, enzymatic proteins (the range is from eleven to thirty depending on the source) in serum. Only a small number of the proteins formed in the liver are reserved for the complement system.
The name was given because this system “complements” or supplements, the action of antibodies. Complement fixation may be complete or partial.
Complete fixation occurs if the foreign antigen is cellular. This type (usually bacteria) causes the complement proteins to bond to the antigen/antibody complex. They then bind to one another, forming an enzymatic ring that punch holes in the cell wall of the antigen, allowing fluid and electrolytes to enter and thus causing the death of the cell. This process is called lysis.
If the foreign antigen is not a cell, a virus for example, only partial fixation takes place when only some of the complement proteins are bound to the antigen/antibody complex. This is called a chemotaxic factor. Chemotaxis means “chemical movement,” which causes the attraction of macrophages to engulf and destroy an antigen.
Interferons are peptides secreted by activated lymphocytes, macrophages, and tissue cells usually infected with viruses. Interferons, as their name implies, “interferes” with further replication of these cells, thereby preventing their spread to surrounding cells, thus causing a resistance to infections.
Interferons stimulate the activities of macrophages and NK (natural killer) cells, thus increasing phagocytosis. They can regulate cell growth and can activate or suppress selected components of the immune system.
Interferons enhance some primary antibody responses while inhibiting others. They are also able to affect the specific cytotoxicity of lymphocytes. It is these abilities of interferons that have prompted scientists to study their possible usefulness in the treatment of cancer.
Interferons are examples of cytokines, chemical messengers released by tissue cells to coordinate local activities. Cytokines are the “hormones” of the immune system, and are released to alter activities of cells and tissues throughout the body. Interferons are not antiviral agents in themselves, but rather act as a stimulant to noninfected cells, causing them to synthesize another protein with antiviral characteristics.
The natural production of interferon is not restricted to viral infections, however. They can respond to such other inducers as Rickettsiae, bacteria, and synthetic polymers.