Antigens and Antibodies - Lesson 48

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An antigen is a substance, usually foreign to the body (such as a poison, virus, or bacterium), that stimulates the production of antibodies. Antibodies are protein substances made by white blood cells in response to the presence of foreign antigens. For example, the flu virus (antigen) enters the body, causing the production of antibodies in the bloodstream. These antibodies will then attach to and destroy the antigens (viruses) that produced them. The reaction between an antigen and an antibody is called an immune reaction (immune/o means protection).

Another example of an antigen-antibody in the Rh condition. A person who is Rh+ has a protein coating (antigen) on his or her red blood cells (RBCs). This antigen factor is something that the person is born with and is normal for him or her. A person who is Rh- has normal RBCs as well, but they do not carry the Rh factor antigen.

If an Rh- woman and Rh+ man conceive an embryo, the embryo may be Rh- or Rh+. A dangerous condition arises only when the embryo is Rh+, during delivery of the first Rh+ baby, some of the baby’s blood cells containing antigens may escape into the mother’s bloodstream. This sensitizes the mother so that she produces a low level of antibodies to the Rh+ antigen. Because this occurs at delivery, the first baby is generally not affected and is normal at birth. Sensitization can also occur after a miscarriage or an abortion.

Difficulties arise with the second Rh+ pregnancy. If the embryo is Rh+ again, during pregnancy the mother’s acquired antibodies will enter the infant’s bloodstream and attack the infant’s RBCs (Rh+). The infant’s RBCs are destroyed, and the infant attempts to compensate for this loss of cells by making new immature RBCs (erythroblasts). The infant is born with a condition known as erythroblastosis fetalis or hemolytic disease of the newborn (HDN). One of the clinical symptoms of erythroblastosis fetalis is jaundice, or yellow skin pigmentation. The jaundice results from the excessive destruction of RBCs, which causes a substance called bilirubin (chemical pigment produced when hemoglobin from the RBCs is broken down) to accumulate in the blood.

To prevent HDN: Rh immunoglobulin is given to the mother at 28 weeks of pregnancy and within 72 hours after each Rh delivery or after every abortion and miscarriage if the father is Rh+. The globulin binds to Rh cells that have escaped into the mother’s circulation, and thus prevents the mother from making Rh+ antibodies. This ensures that future babies will not develop HDN.

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