Blood plasma contains antibodies, a type of protein that can fight a substance considered foreign to the host body. The body manufactures antibodies to correspond to naturally occurring antigens inherited through DNA. Foreign antigens occasionally introduced by way of pregnancy or blood transfusions will also cause the body to make antibodies. Like keys to a lock, antibodies circulate in the blood plasma, ready to defend against their matching antigen counterpart.

At birth, the DNA configures all of the cellular codes that define a person's antigen makeup. If the A antigen is present, for example, then the antibody called anti-A, if infused via a plasma transfusion, will bond with that antigen like a key fits into a lock. Once anti-A has “locked” onto the A antigen, the cell is unable to function and is removed by the body.

Karl Landsteiner discovered that natural blood type antibodies occur in opposites. If the blood is antigen group A, then the circulating plasma antibody will be anti-B. If the blood is antigen group B, the circulating plasma antibody will be anti-A. If the blood is antigen group O, the circulating antibodies will be both anti-A and anti-B. If the blood is antigen group AB, neither anti-A or anti-B will be present in the patient’s plasma.

A pregnant woman can form antibodies in her plasma when her fetus develops its own genetically inherited antigens. If these antigens are “foreign” to the mother, she will form antibodies against them.

The most common antibody formed is the anti-D or anti-Rh factor. Mothers who are Rh negative do not posses the D antigen, and therefore when the baby has it, her body will produce the anti-D antibody.

According to the American Pregnancy Association, women who are Rh negative should receive Rh Immune Globulin at 28 weeks of gestation to prevent formation of the anti-D antibody. If the baby is born Rh or D positive, she should receive another dose of Rh Immune Globulin. The anti-D antibody is the only known preventable antibody in pregnancy.

Other stimulated antibodies that circulate in plasma come from receiving, or exposure to, blood antigens. A blood transfusion from the general population exposes the recipient to foreign blood antigens. The scientists that discovered the antibodies generally had their names associated with the discoveries, and there are hundreds, possibly thousands, of antibody-producing antigens. Karl Landsteiner led the research into plasma antibodies and the corresponding antigens.

Not all plasma antibodies are associated with blood banking and blood typing. Antibodies are also formed to substances like viruses and allergens. Antibodies have been artificially created in humans that are harvested and become vaccines. Some antibodies detected in laboratory blood tests can show that a patient has been infected with the HIV, for instance.