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Isoimmunization

Q: 1) What exactly is meant by isoimmunization against Rh antigen? 2) When is it required to take Rh antigen, that is once we come to know that my wife is pregnant or before when we decide to have a baby?

A:1. The term isoimmunisation refers to the production of antibodies against antigens found (or expressed) in all normal individuals. 2. Several antigens are present (expressed) on red cells. These are referred to as blood groups. Many blood group systems exist and the clinically most important are ABO system and Rh System. An individual may be Rh-positive (i.e. he/she expresses Rh-D antigen) or Rh-negative (i.e he/she does not express Rh-D antigen). It is necessary to understand a few basic principles to comprehend the problems due to Rh status. 1. Antigens and therefore the blood groups are passed on to the child from the parents. Thus if the child Rh-D positive one or both of his/her parents must be Rh-positive. 2. An individual not possessing an antigen (in this case a D-negative individual) can develop antibodies against D-antigen if the D-antigen enters the individual's circulation. Antigens and antibodies interact and destroy the red cells on which the antigens are present. An Rh negative woman (i.e. not possessing Rh-D antigen), can develop antibodies against Rh-D, if she receives Rh-positive blood transfusion. During pregnancy, mostly during delivery, some blood enters the mother's circulation from the fetus. A pregnant women can thus develop antibodies against Rh-D if the blood from an Rh-positive fetus gets in to her circulation. 3. Suppose a woman is Rh-negative and her husband is Rh-positive. Her fetus may be Rh-positive or Rh negative. Suppose she has an Rh-positive fetus. During delivery blood from her D-positive fetus can pass on to her blood. She will develop antibodies against Rh-D antigen. These antibodies can pass from mother to the fetus during next pregnancy. During next pregnancy if she again has an Rh-positive fetus. Anti-D antibodies in her blood will cross the placenta and get into the fetal blood. These antibodies will destroy the blood in the fetus. With each exposure to Rh-positive blood the amount of antibodies formed increases. Therefore the risk goes on increasing to the Rh-positive with each subsequent pregnancy. 4. To avoid this first the Rh-D status of the pregnant women is determined. Her blood is tested. If she is Rh-negative (i.e. does not have Rh-D antigen)the Rh status of the husband is determined. If the husband is Rh-negative no action is taken. Suppose he is Rh-positive, the mother is given anti-D to destroy the fetal cells that may have entered her circulation to prevent her developing anti-D antibodies. 5. Before providing further clarification inquirer should get the Rh status of the wife determined through a simple blood test. If she is Rh-negative the Obstetrician should be consulted for further advice and management.

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