Agranulocytes and Lymphocytes

• Leukocytes are shown in a human blood smear
• Neutrophil = A
• Eosinophil = B
• Basophil with obscured nucleus = C
• Typical monocyte showing vacuolated cytoplasm and cerebriform nucleus = D
• Lymphocyte = E

Agranulocytes

• Agranulocytes include monocytes, macrophages, and lymphocytes
• They contain relatively fewer granules than granulocytes
• Monocytes and macrophages make up the mononuclear phagocyte system (MPS)
• Both monocytes and macrophages participate in the immune and inflammatory response as powerful phagocytes
• They also ingest dead or defective host cells
• Monocytes are immature macrophages
• Monocytes are formed and released by the bone marrow into the bloodstream
• As they mature, monocytes migrate into a variety of tissues and fully mature into tissue macrophages
• Other monocytes may mature out of the vessels in response to infection or inflammation

Lymphocytes

• Lymphocytes constitute approximately 20-40% of the total leukocyte count
• They are primary cells of the immune response
• Most lymphocytes transiently circulate in the blood and eventually reside in lymphoid tissues as mature T cells, B cells, or plasma cells

Natural Killer (NK) Cells

• NK cells resemble lymphocytes and kill some types of tumor cells (in vitro) and some virus-infected cells without prior exposure
• They develop in the bone marrow and circulate in the blood

Platelets

• Platelets (thrombocytes) are not true cells, but disk-shaped cytoplasmic fragments that are essential for blood coagulation and control of bleeding
• They lack a nucleus, have no deoxyribonucleic acid (DNA), and are incapable of mitotic division
• They do contain cytoplasmic granules capable of releasing proinflammatory biochemical mediators when stimulated by injury to a blood vessel
• Platelets on the left are moderately activated, with the beginning of formation of pseudopodia
• Platelets on the right are fully activated, with extensive pseudopodia
• Normal platelet concentration is 150,000 to 400,000 platelets/mm³ of circulating blood
• The normal ranges may vary slightly from laboratory to laboratory
• An additional one third of the body's available platelets are in a reserve pool in the spleen
• A platelet circulates for approximately 10 days, ages, and is removed by macrophages of the spleen

Thrombopoietin (TPO)

• TPO, a hormone growth factor, is the main regulator of the circulating platelet mass
• TPO is primarily produced by the liver and induces platelet production in the bone marrow
• Platelets express receptors for TPO
• When circulating platelet levels are normal, TPO is adsorbed onto the platelet surface and prevented from accessing the bone marrow and initiating further platelet production
• When platelet levels are low, the amount of TPO exceeds the number of available platelet TPO receptors, and free TPO can enter the bone marrow