The C antigen is part of the Rhesus blood group system and can cause incompatibility issues during pregnancy or blood transfusions. Testing for the antigen can be done in a lab. Incompatibility with the C antigen is rare but can cause complications in pregnancy. Serotyping can reveal the presence of different C antigens. No treatment is currently available for incompatibility with the C antigen.
The C antigen is an antigen in the Rhesus blood grouping system. This blood group system is associated with over 40 antigens that may or may not be present in a person’s blood, determining their place in this system. The most famous rhesus antigen is antigen D, sometimes called rhesus factor, which can cause complications during pregnancy. Blood testing for antigen C can be performed in a laboratory that has the necessary serotyping tools to identify the presence of this antigen in the blood.
As with some other blood group antigens, antigen C can create a blood incompatibility that can cause problems during pregnancy or with blood transfusions. If someone does not have this antigen and is exposed to the blood of someone who has it, C antibodies will develop. With subsequent exposure, a clotting reaction will occur in the blood. For this reason, it is important to avoid transfusing C positive blood into people who lack the antigen.
Although the D antigen is responsible for more than 90% of the Rhesus-associated antigenic incompatibility that can occur during pregnancy, it is possible that a maternal-fetal antigenic incompatibility is caused by the C antigen. In this case, the fetus has the antigen and the mother are not. The mother is exposed to fetal blood and develops C antibodies. She is usually able to carry the pregnancy to term, but if she becomes pregnant again with a C positive baby, her immune system will attack the fetus, causing haemolytic disease in the fetus or in the newborn.
If a doctor suspects that antigen incompatibility may be a problem, they may request a serotype of the mother to check for the presence of antigen C. Antigen incompatibility may lead to increased risks during pregnancy. While steps cannot be taken to prevent incompatibility, being aware of the problem can help a doctor act quickly if complications arise during the course of pregnancy. Serotyping can reveal the presence of large C or small C antigen, which are two different antigens present in the Rhesus blood group system.
Since incompatibility with antigen D is well known, when mothers have this incompatibility, they can be treated with a drug called RhoGam® after birth. This drug destroys D antibodies so a mother will not experience an adverse reaction in a future pregnancy. Since incompatibility with antigen C is much rarer, no treatment is available as of 2009.
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