An antigen is any foreign matter containing a protein that enters the body to trigger an antibody response. It may be a virus, bacteria, fungus, snake venom, a splinter or a transplanted organ. In this case it is a sperm.
An antibody is a water-soluble protein produced from globulins (eg. gammaglobulin) in the spleen, lymph nodes, thymus gland, liver or bone marrow in response to an antigen (foreign protein). Antibodies attack antigens to render them inactive and no longer infective.
In particular, antibodies are produced in response to an infection by a virus. The antibodies, once formed, are stored in the spleen and liver, so that when the person is exposed to the same infection a second time, the antibodies are already present and can react immediately to destroy the invading viruses and ward off any infection. This is why most viral infections (eg. chickenpox, measles) can be caught only once.
An antibody may be produced in response to a vaccine that contains selected protein particles from a virus (eg. measles), and thereby give immunity against that particular infection.
Bacteria produce too many different antigens for antibodies to be particularly effective in totally preventing an infection, but they still act to reduce the seriousness of subsequent infections, and eventually destroy the invader.
In uncommon cases, a woman for reasons not yet understood, may develop antibodies against her partner’s sperm, which act as an invading antigen. The sperm are attacked as soon as they enter the uterus, and are therefore prevented from reaching the egg and fertilising it. Once sensitised, in the same way as the response to a viral infection, every subsequent exposure (eg. episode of sex) reinforces the antibody reaction.
This situation is very difficult to diagnose and treat, but intrauterine insemination (IUI) or AIH (artificial insemination by husband) outside the body followed by a GIFT (gamete intrafallopian transfer) overcomes the abnormal immune reaction.