Which of the following prevent$(s)$ blood clotting in uninjured blood vessels?

The most active phagocytic white blood cells are

$A$ person meets with an accident and a great loss of blood has occurred. There is no time to analyze his blood group. It is safe to transfuse blood of group:

In the child of which one of the following couples,Erythroblastosis fetalis is likely to occur?

When the blood of one person is mixed with the serum of another person,in certain cases,clumping of $RBCs$ takes place. This clumping is due to:

Even though a donor $X$ and a recipient $Y$ belong to the same blood group,transfusion of blood leads to agglutination. This is because