To agglutinate red blood cells is to clump them together with an antibody against something in the red cell membrane.

So, if blood cells of type A or type AB are mixed with antibodies against the type A substance, those cells will agglutinate. Likewise, if type B or type AB cells are mixed with antibodies against the type B substance, they will agglutinate. Type O cells are those that do not agglutinate when mixed with anti-A or anti-B antibodies.

If your red blood cells are type A, then you have in your blood antibodies to type B substance. If you're type B, you have antibodies to type A substance. If you're type AB, you don't have either kind of antibody, and if you're type O, your blood has antibodies to both type A and type B cells.

Hope this helps.

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