Antibodies, also called immunoglobulin, are protective, non-enzymatic proteins produced by the immune system in response to the presence of antigens.

Antibodies, also known as immunoglobins, are protein molecules that are involved in the immune defense of the body. Antibodies bind to specific antigens on pathogens, coat these extracellular pathogens, and neutralize them by blocking key sites on the pathogen that enhance their infectivity. Antibody neutralization can prevent pathogens from entering and infecting host cells, as opposed to the cytotoxic T-cell-mediated approach of killing cells that are already infected to prevent the progression of an established infection. The neutralized antibody-coated pathogens can then be filtered by the spleen and eliminated in urine or feces.

Not all antibodies bind with the same strength, specificity, and stability. In fact, antibodies exhibit different affinities (attraction) depending on the molecular complementarity between antigen and antibody molecules. An antibody with a higher affinity for a particular antigen would bind more strongly and stably. It would be expected to present a more challenging defense against the pathogen corresponding to the specific antigen.

Key Points

• A special protein known as antibodies or immunoglobulin provide an immune defense to the body from antigens.

• Antibodies bind to specific antigens on pathogens; this binding can inhibit pathogen infectivity by blocking key extracellular sites, such as receptors involved in host cell entry.

Key Terms

immunoglobulin: any of the glycoproteins in the blood that respond to invasion by foreign antigens and that protect the host by removing pathogens; also known as an antibody.

antigen: a substance that binds to a specific antibody; may cause an immune response

immunity: resistance against disease-causing or harmful foreign substances

ligand: a molecule that binds to another (usually larger) molecule

pathogen: a bacterium, virus, or other microorganism that can cause disease