The eosinophils, which are often present in large numbers in the presence of a parasitic infection, have numerous red-staining granules. Agranulocytes Lymphocytes are smaller than granulocytes, are nearly all nucleus, and are nearly as common as neutrophils. The cytoplasm can be seen as a thin area of lighter blue between the nucleus and the cell membrane. They are responsible for the initiation of the immune response and for the acquisition of immunity. Monocytes, the other type of agranulocyte, are much larger than lymphocytes, with a greater ratio of cytoplasm to nucleus.
Macrophages consume foreign substances, such as bacteria, by phagocytosis.
The stem cells from which red cells and leukocytes are developed are found in the bone marrow. Also in the bone marrow are giant cells called megakaryocytes. Megakaryocytes disintegrate before enter the bloodstream, so that only the fragments, called platelets, remain. When platelets encounter a damaged surface they swell and stick to the surface and each other to form a plug. The plug seals blood vessels, allowing time for the slower clotting process to take place and form a tight seal.