Agranulocytes and Mononuclear Phagocytes

Agranulocytes

• Single non-lobulated nucleus
• Granules in cytoplasm are too small to see
• Two types based on structure, not lineage: Lymphocytes and Monocytes

Monocytes

• Largest leukocytes
• Bluish cytoplasm and a large C-shaped nucleus
• Highly motile and phagocytic
• Travel through bloodstream to reach connective tissues, where they transform into macrophages (large phagocytic cells)
• Precursor cells of macrophages, osteoclasts, microglia, and other cells of the mononuclear phagocyte system in connective tissue
• All monocyte-derived cells are antigen-presenting cells
• Monocytes and macrophages are the same cells at different stages of maturation

Mononuclear Phagocytic System

• Includes: Microglia (CNS), Kupffer Cells (Liver), Macrophages (Bone marrow, Connective tissue, Dust cells [Lung]), Dendritic cells (Lymph node, Spleen), Osteoclasts (Bone resorption), Langerhans cells (Epidermis)
• All cells have phagocytic activity

Lymphocytes

• Smallest leukocytes
• Round nucleus occupies most of cell volume
• Cytoplasm is light clear blue
• Increased numbers are commonly seen in viral infections
• Lymphocytes vary in life span according to their specific function, some live for a few days and some live for many years

Lymphocyte Types

• T cells: Directly kill foreign or infected cells, activate phagocytes to destroy microbes (cell mediated immunity)
• B cells: Differentiate into plasma cells, secrete antibodies that bind to specific antigens and mark them for destruction by phagocytic cells (humoral immunity, long term immunity)

Lymphopoiesis

• The process by which lymphocytes are formed
• Precursor cells in bone marrow
• T Lymphocytes: Travel to the Thymus and then directly into blood
• B Lymphocytes: Remain in bone marrow
• Natural killer lymphocytes: Direct lineage from bone marrow

Morphological Classification of Lymphocytes

• The amount of cytoplasm depends upon the state of activity of the lymphocyte
• In circulating blood, there are mostly small, inactive lymphocytes
• Large lymphocytes (9-18 µm) are active lymphocytes
• Small lymphocytes (6-9 µm) are inactive lymphocytes
• Different cell types express specific cell surface proteins (including CD4 or 8)

Immunohistochemistry

• Direct method