C antigen is an antigen in the Rhesus blood grouping system. This blood group system is associated with over 40 antigens which may or may not be present in a someone's blood, determining her or his place in this system. The most famous Rhesus antigen is the D antigen, sometimes called the Rhesus factor, which can cause complications in pregnancy. Blood analysis for C antigen can be performed in a lab which has the serotyping tools necessary to identify the presence of this antigen in the blood.
As with some other antigens associated with blood types, the C antigen can create a blood incompatibility which may cause problems during pregnancy or with blood transfusions. If someone does not have this antigen and she or he is exposed to the blood of someone who does, C antibodies will develop. With the next exposure, a coagulation reaction will occur in the blood. For this reason, it is important to avoid transfusing C positive blood into people who do not have the antigen.
Although the D antigen is responsible for over 90% of the Rhesus-associated antigen incompatibility that can occur during pregnancy, it is possible for a maternal-fetal antigen incompatibility to be caused by C antigen. In this case, the fetus has the antigen and the mother does not. The mother is exposed to fetal blood, and develops C antibodies. Usually she is able to carry the pregnancy to term, but if she gets pregnant again with a C positive baby, her immune system will attack the fetus, causing hemolytic disease in the fetus or newborn.
If a doctor suspects that antigen incompatibility may be an issue, she or he may request a serotype of the mother to check for the presence of the C antigen. Antigen incompatibility can result in increased risks during the pregnancy. While steps cannot be taken to prevent the incompatibility, being aware of the issue can help a doctor act quickly if complications emerge during the course of the pregnancy. Serotyping can reveal the presence of big C or little c antigen, which are two different antigens found in the Rhesus blood group system.
Because D antigen incompatibility is well understood, when mothers have this incompatibility, they can be treated with a drug called RhoGam® after birth. This drug destroys the D antibodies so that a mother will not experience an adverse reaction in a future pregnancy. Because C antigen incompatibility is much more rare, no treatment is available as of 2009.