Antigen

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When an antigen is introduced into the body it causes the production of antibodies. Antigens can include bacteria, cells of transplanted organs, and toxins. Strictly speaking, an antigen is any substance that can stimulate the production of antibodies and combine specifically with them. For example, antigen A stimulates production anti-B antibodies, and antigen B stimulates production of anti-A antibodies. It is important to note that antigens stimulate production of antibodies and do not directly produce produce them. Antibodies are what determine whether or not a blood transfusion recipient will have an adverse reaction to the donor's blood. A person with blood type B for example, will have anti-A antibodies (reject blood type A as well as blood type AB-due to the presence of antigen-A in blood type AB) and as a result, could only safely accept blood from a donor with blood type B or blood type O (O negative blood types are considered the universal donor).