Cyanocobalamine is essential for the formation of

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:

Match the columns:

Column-$I$ (Blood group)	Column-$II$ (Antibody)
$A$. $A$	$P$. $b$
$B$. $B$	$Q$. $a$
$C$. $AB$	$R$. $nil$
$D$. $O$	$S$. $a$ and $b$

Prothrombinase is formed in the presence of

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

After examining the blood groups of a husband and wife,the doctor advised them not to have more than one child. The blood groups of the couple are likely to be: