WBCs as they are called have a nucleus, which is often quite large and lobed. They can move around and can squeeze out through the walls of blood capillaries into all parts of the body. Their role is to fight infection, and to clear up any dead body cells. Platelets are small fragments of cells, with no nucleus. They are made in bone marrow.
Platelets help in the formation of blood clots.
When platelets come into contact with a damage tissue, they stick to the edges of the damaged area, and then to each other, forming a plug. Larger wounds however need a larger barrier than this. Blood plasma contains several substances, which are involved in blood clotting. There are thirteen of these blood-clotting factors. If any one of them is defective, then blood will not clot.