Which organ is known as the 'graveyard of red blood cells'?

The most well-known blood grouping system is the $ABO$ system. It is named $ABO$ and not $ABC$ because the $\text{O}$ in it refers to having:

Identify the cells and their functions based on the provided figure:
$(a)$ Fig. $A$ - Megakaryocyte - formation of platelets
$(b)$ Fig. $B$ - Lymphocyte - synthesis of antibodies
$(c)$ Fig. $A$ - Eosinophil - secrete antihistamine
$(d)$ Fig. $A$ - Neutrophil - secrete heparin

Hemoconia are -

Blood group $O$ has:

$A$ significant decrease in the number of lymphocytes due to a deficiency of folic acid is known as: