Granulocytic leukocytes can be further subdivided on the basis of morphology into neutrophils, eosinophils, and basophils. Each of these begins as a multipotential stem cell in the bone marrow.

 Neutrophils

Neutrophilic leukocytes, particularly the polymorphonuclear (PMN) type , provide an effective host defense against bacterial and fungal infections. The antimicrobial function of PMNs is essential in the innate immune response. Although the monocytes-macrophages and other granulocytes are also phagocytic cells, the PMN is the principal leukocyte associated with phagocytosis and a localized inflammatory response. The formation of an inflammatory exudate (pus), which develops rapidly in an inflammatory response, is com posed primarily of neutrophils and monocytes.

PMNs can prolong inflammation by the release of soluble substances, such as cytokines and chemokines. The role of neutrophils in influencing the adaptive immune response is believed to include shuttling pathogens to draining lymph nodes, antigen presentation, and modulation of T helper types 1 and 2 responses. Functionality of neutrophils is no longer considered as limited as it once was because new research has discovered that PMNs have a 5.4 day lifespan.

Mature neutrophils are found in two evenly divided pools, the circulating and marginating pools. The marginating granulocytes adhere to the vascular endothelium. In the peripheral blood, these cells are only in transit to their potential sites of action in the tissues. Movement of granulocytes from the circulating pool to the peripheral tissues occurs by a process called diapedesis (movement through the vessel wall). Once in the peripheral tissues, the neutrophils are able to carry out their function of phagocytosis.

The granules of segmented neutrophils contain various anti bacterial substances (Table 1). During the phagocytic process, the powerful antimicrobial enzymes that are released also disrupt the integrity of the cell itself. Neutrophils are also steadily lost to the respiratory, gastrointestinal (GI), and urinary systems, where they participate in generalized phagocytic activities. An alternate route for the removal of neutrophils from the circulation is phagocytosis by cells of the mononuclear phagocyte system.

Table1. Function and Types of Granules in Neutrophils

Eosinophils and Basophils

Although capable of participating in phagocytosis, eosinophils and basophils possess less phagocytic activity. The ineffective ness of these cells results from the small number of cells in the circulating blood and lack of powerful digestive enzymes. Both eosinophils and basophils, however, are functionally important in body defense.

 Eosinophils

 The eosinophil is considered to be a homeostatic regulator of inflammation. Functionally, this means that the eosinophil attempts to suppress an inflammatory reaction to prevent the excessive spread of the inflammation. The eosinophil may also play a role in the host defense mechanism because of its ability to kill certain parasites.

A functional property related to the membrane receptors of the eosinophil is the cell’s ability to interact with the larval stages of some helminth parasites and damage them through oxidative mechanisms. Certain proteins released from eosinophilic granules damage antibody-coated Schistosoma parasites and may account for damage to endothelial cells in hypereosinophilic syndromes.

Basophils

Basophils have high concentrations of heparin and histamine in their granules, which play an important role in acute, systemic, hypersensitivity reactions . Degranulation occurs when an antigen such as pollen binds to two adjacent immunoglobulin E (IgE) antibody molecules located on the surface of mast cells. The events resulting from the release of the contents of these basophilic granules include increased vascular permeability, smooth muscle spasm, and vasodilation. If severe, this reaction can result in anaphylactic shock.

A class of compounds known as leukotrienes mediates the inflammatory functions of leukocytes. The observed systemic reactions related to leukotrienes were previously attributed to the slow-reacting substance of anaphylaxis.

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