Question

why there is no coagulation when blood group a and o are mixed(considering that o blood group has antibody a)?

Answer 1

During transfusion two types of compatibility procedures are performed.
1 Red blood cell compatibility
2) Plasma compatibility
Blood group A has, A antigen on RBCs and antibody B in its plasma. O blood group has no antigen on RBCs, while it contain antibody A and B in plasma part.
If a person with blood group A is given RBCs of O blood group then this transfusion will not show any coagulation since it does not contain any antigen but if this person is given both RBCs and plasma, then the transfusion will result into coagulation because antibody A present in O blood group will react with antigen A and cause coagulation.
In simple words, if whole blood (plasma and cells) of O type is mixed with A type then only coagulation will occur.