Blood does not clot inside the blood vessels due to the presence of

Which blood group is the universal donor and which is the universal recipient?

Name the blood cells whose reduced count causes excessive loss of blood from the body due to a defect in the blood clotting process.

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:

Write a short note on $ABO$ blood grouping.

Mammalian $RBCs$ are..... in shape.