Cells involved in inflammation

Neutrophils (also known as polymorphonuclear neutrophils), are the predominant cells in acute inflammation as well as in abscess formation, lo-culation, and empyema. They are the white blood cells (WBCs) most responsible for the leukocytosis that occurs in response to an inflammatory or infectious crisis. Neutrophils are granular leukocytes with a multilobate nucleus and fine cytoplasmic granules that stain readily with neutral dyes. In the inflammatory response, neutrophils are the first cells to arrive at the injured area. The major activity of neutrophils is phagocytosis of invading bacterial cells, with subsequent destruction of the cells through the release of lysosomal enzymes.

Eosinophils (eosinophilic granulocytes) have a characteristic bilobate nucleus and cytoplasmic granules that stain orange with Romanovsky's stain and red-orange with eosin. The granules contain hydrolytic enzymes (e.g., histaminase, which inactivates histamine; arylsulfatase B, which inactivates SRS-A). The granules also contain a poorly understood major basic protein. Although they also can be found in peripheral blood, a number of the body's eosinophils exist in hypersensitivity sites within the tissues, where they can abort hypersensitivity reactions. Eosinophils are increased in the peripheral blood in the presence of allergy and parasitic infestation. Eosinophils are

readily chemotactic upon the release of eosinophil chemotactic factor (ECF) from IgE-sensitized mast cells—an occurrence in anaphylaxis. Eosinophils are also phagocytic, although phagocytosis is a minor function.

Basophils (basophilic granulocytes) contain granules that stain blue with Wright's stain. The granules contain histamine, heparin, and slow-reacting substance of anaphylaxis. Basophils are involved in type I immediate, or immunoglobulin E (IgE)-mediated hypersensitivity reactions. When an IgE-specific antigen enters the body, basophils stimulate the formation of IgE, which binds to the surface of the antigen. The basophilic granules then release histamine and other vasoactive substances to produce anaphylactic reactions in susceptible persons. Basophils also play a role in type IV (i.e., delayed) hypersensitivity reactions, such as contact dermatitis.

Macrophages. The mononuclear phagocyte system (also known as the monocyte-macrophage system and reticuloendothelial system) is an extensive network of macrophages that exists throughout the body. Pulmonary alveolar macrophages, Pleural and peritoneal macrophages, Kupffer cells of the liver, Histiocytes of mesenchymal and connective tissue, Mesangial cells of the kidney, Both fixed and mobile macrophages in the lymph nodes, spleen, and bone marrow. Macrophages in the body tissues develop from monocytes that have left the peripheral blood. The

monocytes originally derive from bone marrow precursors. Monocytes in the bone marrow and the peripheral blood can be converted rapidly into additional macrophages when needed.

Macrophages dispose of noxious matter within tissues, for example, microorganisms and necrotic tissue or other debris. Macrophages also appear to serve in tumor cell killing. In phagocytosis, the cytoplasmic membrane extends around particles and engulfs them, forming an intracellular vacuole. In pinocytosis, the cell membrane engulfs extracellular fluid along with the particles. The lysosomes of macrophages contain degradative substances similar to those in neutrophils. Macrophages have surface receptors for the Fc segment of the immunoglobulin G (IgG) molecule and for complement component C3b. These aid the macrophage in phagocytosis of opsonized microorganisms.

Macrophages are important components of the immune system. Their involvement begins with the initiation of the immune response, and they interact closely with T-lymphocytes. B-cell activation requires IL-1, which is secreted by macrophages (and some other cells). B-cell activation also requires that antibody on the B-cell surface match its specific antigen. Antigen on the macrophage surface can serve this purpose.

Mast cells resemble basophils in both structure and function.

Whereas basophils are present mainly in the peripheral blood and at sites of inflammation, mast cells are connective tissue cells found close to small blood vessels. Mast cells contain numerous granules that stain metachromatically with basic dyes. Like basophilic granules, mast cell granules release histagune, heparin, and SRS-A during type I reactions. In addition, mast cell granules release. Agents that cause inflammation (e.g., physical factors, drugs, immunoglobulins, complement components C3a and C5a, cationic proteins) may cause histamine release from mast cells.

Lymphocytes and their derivatives are found in the tissues in all types of inflammation, especially after the acute ingress of neutrophils. All lymphocytes are derived from bone marrow stem cells. Stem cells differentiate into lymphocytes in the primary lymphoid organs (thymus and bone marrow). From these locations, some lymphocytes migrate — via the circulation — to secondary lymphoid organs, namely, the spleen, lymph nodes, and lymphoid germinal centers throughout the body.

Lymphocytes are divided into two types — T-cells and B-cells — which serve different functions (See Immunopathology).

ACUTE INFLAMMATION

Morphological manifestations of inflammation depend upon a number of factors and processes. They are factors of the organisms and the host, type of exudation, cellular proliferation.