Todd Gubin, age 13, of Ottawa, Ontario, Canada, for his question:

HOW IS YOUR BLOOD TYPE DETERMINED?

Your blood is classified into various types or groups based on the presence or absence of certain antigens on the membrane of the red blood cells. If certain antibodies are mixed with blood, they bind themselves to these antigens and cause the red blood cells to clump together. Such clumping can block small blood vessels and result in serious illness.

Blood types play an important role in blood transfusions. A patient who receives blood of a different type than his own might have antibodies that bind themselves to the donor's erythrocyte antigens. The donor's red blood cells would then clump in the patient's blood vessels.

The presence or absence of certain protein substances in the red cells and plasma determine whether your blood is type A, B, AB or O. Blood type is inherited.