Antigens are found on the surface of foreign substances. Antigens are capable of causing a specific immune response and reacting with the products of that response, as with antibody or specific T cells. For instance, lymph nodes are found throughout the body and function as a prime immunologic filter, especially for the fluid known as lymph.
Composed mostly of T cells, B cells, dendritic cells, and macrophages, the lymph nodes strain fluid from most of our tissues, filtering out antigens from the lymph before allowing it back into circulation. In the spleen, the macrophages and dendritic cells capture antigens and present them to T and B cells thus, initiating an immune response.
Antigens in blood transfusions:
Plasma contains antibodies that may cause the red cells of a donor’s blood to clump together (agglutination). Any evidence of agglutination indicates that the donor blood is incompatible with that of the recipient. Agglutination reactions cause the red blood cells to rupture releasing their hemoglobin (hemolysis) which could then clog the kidneys causing death. These reactions are determined by antigens on the red cell membranes.
There are many types of antigens but only two groups are likely to cause such serious reactions. These are the A and B antigens and the Rh factor. The letters of the blood type indicate the type of antigens present on the red cells.
Type A blood has different antigens than Type B. Type AB blood contains antigens of both Type A and Type B. Type O has no antigens. A person with Type A blood cannot receive Type B or Type AB blood without having a violent immune response. However, a person with Type A blood can receive blood from another type A or the neutral Type O without having an immune response.
An individual’s blood type is determined by heredity. Persons with Type O blood are said to be universal donors because they lack the AB red cell antigens and, in an emergency, their blood can be given regardless of the blood type of the recipient. Type AB individuals are called universal recipients because their blood contains no antibodies to cause harm.
A vast majority of the population has another red cell antigen group called the Rh factor. Such individuals are referred to as Rh positive. Those who lack this protein are said to be Rh negative. If Rh positive blood is given to an Rh negative person, the recipient can become sensitized to the protein on the Rh positive blood cells. Antibodies will then be produced to the “foreign” Rh antigens and destroy the transfused red cells.
This is particularly disturbing during pregnancy when an Rh positive fetus receives the mother’s anti-Rh antibodies, causing the destruction of the fetus’s red cells. This is prevented by the use of RhoGAM when these pre-formed antibodies clear the mother’s circulation of antigens and thus preventing the activation in her immune system against her developing fetus.
