Question

If Blood O can be mixed with Blood A i.e O donating to A, then why can't A be mixed with O i.e. A donating to O. Basically If we can put O into A then we must is able to put A into O. But we know the second case isn't possible. Why so?
And if I kind of reframe my question then it goes as when Blood O is donated to A then why doesn't the antibody A in Blood O coagulate with Antigen A in Blood A?

Answer 1

Blood group O has antibodies against antigens A and B (Anti A antibody,Anti B antibody ). Blood group A has A antigen. If someone with blood group A receives donor blood with group O, then anti-A antibodies in the donor blood should attack the A antigens in the recipient's blood.

In reverse, Group A or Group B recipient's plasma contain soluble A or soluble B (antigen not bound to red cell surface). These free antigens will bind to the antibodies from the donors and prevent it to react on the red cell surface antigen A or B. The other thing is usually universal O should be low titer Anti-A or Anti-B.