What Is C Antigen?
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 C 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 the C 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.
Written by S.E. Smith
