Antigen

Immunoglobulin G antibody molecule. Computer model of the secondary structure of immunoglobulin G (IgG). This is the most abundant immunoglobulin and is found in all body fluids. Each Y-shaped molecule has two arms (top) that can bind to specific antigens, for instance bacterial or viral proteins. In doing this they mark the antigen for destruction by phagocytes, white blood cells that ingest and destroy foreign bodies. Antibodies can also kill some pathogens directly, and can neutralise toxins. ALFRED PASIEKA/SCIENCE PHOTO LIBRARY/GETTY IMAGES

What is an Antigen?

Antigens are molecules that stimulate an immune response when introduced into the body. Almost any large molecule can be an antigen, It all depends on  how it is taken in. The body responds to an antigen by making antibodies that react against it. Those antibodies can help the body protect itself by recognizing the antigen -- and any virus, bacteria, etc. that it is attached to -- whenever it shows up again.

Then those same antibodies initiate the fight to get rid of the invader.

The first time the body is exposed to a new antigen, building an immune response takes time. The body has to build an antibody that can recognize it. That requires rearranging genes by trial and error -- a process that takes time. However, once a good antibody has been produced the first time, the process is faster. The body keeps the machinery it needs to reproduce it ready to go. Antibodies can therefore be produced much more quickly after the body has already been taught to recognize an antigen (after an initial infection or vaccination). Vaccines work by telling the body to make antibodies against a disease. That way, if a person is ever exposed to that disease, their immune system will already be ready to fight back.

More specifically, vaccines use a substance known as an adjuvant. Adjuvants stimulate the body to produce an immune response against an antigen that is part of the pathogen the vaccine is designed to protect against.

Usually the antigen is a surface protein from the bacterium or virus in question.

What are the Antigens Used in  HPV Vaccines?

HPV vaccines use surface proteins from the human papillomavirus to stimulate an immune response. The currently available vaccines use the same basic antigen. The antigen is virus like particles made from the L1 capsid (outer shell) protein of various HPV strains.

However, they have different adjuvants.

Other than the adjuvants, the main reason the HPV vaccines differ is the number and type of strains of virus they are designed to protect against. Cervarix is designed to protect against two high risk strains. Gardasil protects against two high- and two low- risk strains. Gardasil 9 protects against seven high risk strains and two low risk strains. That means Gardasil 9 uses more antigens to stimulate the body than the other two vaccines. 

Sources:

Deschuyteneer M, Elouahabi A, Plainchamp D, Plisnier M, Soete D, Corazza Y, Lockman L, Giannini S, Deschamps M. Molecular and structural characterization of the L1 virus-like particles that are used as vaccine antigens in Cervarix™, the AS04-adjuvanted HPV-16 and -18 cervical cancer vaccine. Hum Vaccin. 2010 May;6(5):407-19.

Siddiqui MA, Perry CM. Human papillomavirus quadrivalent (types 6, 11, 16, 18) recombinant vaccine (Gardasil). Drugs. 2006;66(9):1263-71

Merck Canada (2015) Gardasil 9 (Product Monograph). Accessed 1/5/16 at http://www.merck.ca/assets/en/pdf/products/GARDASIL_9-PM_E.pdf

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