Blood cells have antigens on the surface and antibodies in the plasma
Group A: A antigens on surface. Anti B antibodies in plasma
Group B: B antigens on surface. Anti A antibodies in plasma
Group AB: A and B antigens on surface. No antibodies in plasma
Group O: No antigens. Anti A and B antibodies in plasma
Group O is the universal donor as they have no antigens for the antibodies to attach to therefore they won't be rejected by another blood type
They however have anti-a and anti-b antibodies therefore they need to recieve blood with no antigens as if they recieve antigens a or b, the anti a and anti b antibodies will attach to them and this causes agglutination which is bad. O is the only type with no antigens.
Group A can recieve from Group O and A as A has the same antigens and antibodies as the current blood anyway so there will be no agglutination and the blood won't be rejected. This is as the anti-B antibodies can attach to the A antigen. They can donate blood to group AB as AB has no antibodies and A and B antigens so giving more A antigens to attach to no antibodies won't cause a problem
Group B can recieve from Group B and O and it can send to Group B and Group AB
Group AB can recieve from anybody- universal recipients as they don't have any anti bodies but have both A and B antigens so can recieve both.
Group O. Recieve from Group O. Donate to any group
Group A. Recieve from Group A and Group O. Donate to Group AB and A
Group B. Recieve from Group B and Group O. Donate to Group AB and B
Group AB. Recieve from any group. Donate to Group AB
Clotting is as a result of platlets:
Platlets stiumlate clotting. It's an enzyme controlled reaaction. Fibrinogen, in plasma reacts to form fibrin and this forms a mesh which platlets fit into forming a scab. This acts as a barrier and prevents red blood cells from escaping.