Physiology Topics
Substances that are recognized as foreign and provoke immune responses are called antigens. They provoke an immune response by stimulating the production of specific antibodies. Antibodies bind and inactivate specific antigens. Sometimes the immune system fails to distinguish self from non-self, resulting in an autoimmune disorder.
Entire microbes or parts of microbes may act as antigens, as may other proteins from incompatible blood cells, transplanted organs, environmental agents like pollen or some foods. They are usually large, complex proteins, but may also be lipoproteins, glycoproteins or polysaccharide. The small, specific portions of antigens that trigger immune responses are called epitopes.
The surface of red blood cells contain genetically determined glycoprotein and glycolipid antigens that are recognized as foreign if given to a patient with a different blood type, triggering an immune response. Whole blood transfusions must be from the same blood type as the patient. A blood transfusion will be needed to survive after >80% blood loss. If blood loss is minor, only the volume needs to be replenished by infusion of saline or plasma.
The ABO blood grouping is based on the red blood cell antigens A and B. For example, people with type A blood have A antigens and anti-B antibodies, and will have an immune reaction to a blood transfusion containing the B antigen. All the ABO types are described below:
| Blood Type | Antigens | Antibodies | Donor to | Can receive |
| A | A | B | A and AB | A or O |
| B | B | A | B and AB | B or O |
| AB | A and B | none | none | any |
| O | none | A and B | All | O |
The Rh blood grouping is based in the presence of the Rh antigen on red blood cells. People with the antigen are Rh+, people without it are Rh-. Plasma does not have anti-Rh antibodies until an Rh- person receives an Rh+ transfusion, or is pregnant with an Rh+ fetus AND maternal and fetus bloods somehow mix (does not always happens). If a second transfusion or pregnancy put the Rh patient's blood in contact with Rh+ blood, the previously formed Rh antibodies will cause hemolysis of the donated or fetal blood.
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