What Are Antibodies and Antigens?

3D ribbon conformation of the antibody IgG2
Tim Vickers/Public Domain/Wikimedia Commons

An antibody, also known as an immunoglobin, is a Y-shaped protein secreted by certain types of white blood cells which have the ability to identify pathogens (infective agents) such as viruses and bacteria. The two tips of the "Y" are able to latch onto either the pathogen or infected cell at a unique target called the antigen (also known as the antibody generator).

In doing so, the antibody effectively marks the pathogen for neutralization, either by killing it or preventing it from entering a healthy cell, or by signalling other complementary proteins to surround and devour the invader in a process called phagocytosis [from the Ancient Greek for "to devour" (phagein) and "cell" (kytos)].

Antibodies are produced by white blood cells called B-lymphocytes, or B-cells. During the prenatal (before birth) and neonatal (newborn) stages of life, antibodies are passed from the mother to the infant through a process called passive immunization. From there, the child will begin to independently produce antibodies, either in response to a specific antigen (adaptive immunity) or as part of the body's natural immune response (innate immunity).

Humans are capable of producing over ten billion types of antibodies, each defending against a specific type of antigen. The antigen-binding site on the antibody called the paratope is located at the tips of the "Y" and locks onto a complementary site on the antigen called the epitope. The high variability of the paratope allows the immune system to recognize an equally wide variety of antigens.

HIV Antibodies and Antigens

When an HIV infection occurs, measurable HIV antibodies are produced in response to antigens within a week or two of exposure. The antibodies are generated in response to different viral antigens: the p24 antigen, which is generally the first to appear; and the gp120 and gp41 antigens, which are both found on the surface of the virus.

Once infected, the antibodies persist for life and provide the traditional target for HIV antibody tests (including commercially available in-home tests). Fourth-generation combination tests are now capable of detecting both HIV antibodies and p24 antigen, providing speedier, more accurate confirmation of a person's HIV status. 

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Article Sources

  • Janeway, C.; Travers, P.; Walport, M.; and Shlomchik, M. Immunobiology, 5th Edition - The Immune System in Health and Disease. 2001; Garland Science; New York City; ISBN-10:0-8153-3642-X.
  • U.S. Food and Drug Administration (FDA). "FDA approve first rapid diagnostic test to detect both HIV-1 antigen and HIV-1/2 antibodies." Silver Spring, Maryland; a press release issued August 8, 2013.
  • U.S. Food and Drug Administration (FDA). "June 18, 2010, Approval Letter  - ARCHITECT HIV Ag/Ab Combo." Silver Spring, Maryland; issued December 22, 2009.