Explain Antigens ?
A blood type, or group, is based upon the presence or absence of certain substances called antigens on the surface of the red blood cells. Antigens are substances that stimulate cells to produce antibodies. Antibodies are proteins that are produced to help cells to counter substances foreign to the body, such as antigens, by combining with them in the blood to cause agglutination, or clumping. Antibodies in this way are able to inactivate a foreign substance that threatens the well-being of the body.
Blood Groups : The most familiar types of antigens are A, B, and O, constituting the ABO system, discovered in 1900 by Karl Landsteiner. The ABO system is an example of a trait which is inherited by multiple alleles. Groups A and B are inherited as codominant traits, and both are dominant to Group O.
Group O contains neither antigens for A or B, so AO belongs to Group A and BO belongs to group B. Serum from Group A contains antibodies against antigen B; serum from Group B contains antibodies against A. Serum from AB contains neither antibody, and serum from Group O contains both A and B antibodies. Since blood from Group O individuals contains neither antigens, Group O individuals are called universal donors, and since the serum from AB individuals contains neither antibody, AB individuals are called universal recipients.
Rh Factor : The second most important human blood antigen is called the Rh factor. It is so named because it was first discovered in Rhesus monkeys, and later demonstrated in humans. Rh antigen stimulates formation of antibodies in the blood of individuals who do not have the dominant allele for the Rh factor; this includes 15% of the population of the United States. Persons with the Rh antigen are called Rh positive, others are Rh negative. If an Rh negative person is transfused with Rh positive blood, agglutination will occur.
During pregnancy, sensitization to the Rh factor may occur if the fetus is Rh positive and the mother is Rh negative. If cells leak from the fetus to the mother, the mother produces antibodies to the Rh factor. If the antibodies enter the fetus, hemolysis or breakdown of fetal red blood cells called erythroblastosis fetalis may occur. In such cases, the fetus may become jaundiced, or it may die. If a jaundiced child is born alive, it may need an immediate transfusion of Rh positive blood. Such sensitization rarely occurs with the first child, but can occur with the second child. In recent years, Rh negative mothers are given anti-Rh antibodies shortly after the birth of an Rh positive child. These antibodies destroy the Rh antigens in the mother's blood before her body makes antibodies, so that no antibodies reach the blood of the second child.