Which ion is essential for blood clotting?

$A$ person with an unknown blood group ($ABO$ system) has lost a large amount of blood in an accident and needs an immediate blood transfusion. $A$ friend of the person has a valid certificate of their own blood group and is ready to donate blood without delay. What could be the blood group of the donor friend?

The function of the human spleen is to

In the human population,what percentage of individuals possess the $Rh$ antigen (in $,\%$)?

Explain different types of blood groups and donor compatibility by making a table.

Which of the following are agranulocytes?