Overview: Cells of the immune system

Macrophages are the major resident phagocytic cell in connective tissue. They are derived from blood-borne monocytes that migrate into the tissue. Macrophage nuclei are oval to elongated and frequently indented, reflecting the cell’s origin from a monocyte. Macrophages are major effectors of innate immunity and also process and present antigens to lymphocytes to initiate an adaptive immune response. They also secrete chemotactic factors to recruit additional cells to infection sites. 1000x

Mast cells are produced in the bone marrow, enter the circulation and then become fixed, resident cells in connective tissue. These cells are spherical or elongated with a centrally-located, spherical nucleus. Their cytoplasm is filled with numerous granules that contain mediators of the inflammatory response and allergic reactions, including heparin, histamine and protease enzymes. H & E, Toluidine blue, 1000x

Neutrophils, or polymorphonuclear (PMN) leukocytes, are highly motile, granular white blood cells that migrate in large numbers from blood (left image) into the connective tissue at sites of infection. Each cell has a distinctive, 3-5 lobed nucleus. Neutrophils are phagocytic and remove pathogens and cellular debris. Accumulation of dead neutrophils, bacteria and tissue debris form the whitish-yellow exudate called pus. They also secrete chemotactic factors to recruit additional cells to sites of infection. 1000x

Eosinophils are granular white blood cells that migrate from the blood (left image) into connective tissues. They are phagocytic and function in parasitic infections. Eosinophils have intensely eosinophilic granules in their cytoplasm and a bi-lobed nucleus. 1000x

Lymphocytes are the main effector cells of the adaptive immune response. They are small, spherical white blood cells with rounded, highly heterochromatic nuclei and minimal, agranular cytoplasm. Lymphocytes are classified as either T or B lymphocytes but can only be distinguished by immunohistochemical staining methods. Thymus-derived (T) lymphocytes provide cell-mediated immunity and consist of a number of subtypes including natural killer cells; B lymphocytes transform into plasma cells after entering the connective tissue and secrete antibodies. Both types form memory cells.

Plasma cells differentiate from activated B lymphocytes and reside in connective tissues. Plasma cells secrete antibodies, (immunoglobulins) that specifically recognize and bind to antigens. Plasma cells are considered to have limited migratory ability.