Erythrocytes, leucocytes and platelets are collectively called formed elements of the blood. Erythrocytes are the red blood cells (RBC). These molecules play a significant role in transport of respiratory gases. Leucocytes are the white blood cells (WBC). There are two main categories of WBCs granulocytes and agranulocytes. Neutrophils, eosinophils and basophils are different types of granulocytes, while lymphocytes and monocytes are the agranulocytes.
Neutrophils and monocytes are phagocytic cells which destroy foreign organisms entering the body. Basophils secrete histamine, serotonin, heparin etc. and are involved in inflammatory reactions. Lymphocytes are responsible for immune responses of the body. Platelets also called thrombocytes,.are cell fragments produced from megakaryocytes. Platelets are responsible for clotting or coagulation of blood.