Figure 518

Electron micrograph of a mast cell. The cytoplasm is virtually filled with granules. Note a small lymphocyte present in the upper left of the figure. x6,000.

filled with large, intensely basophilic granules (Fig. 5.18). The mast cell is related to, but not identical with, the basophil, a blood cell that contains similar granules (Table 5.6). The cell surface exhibits numerous microvilli and folds. The cytoplasm displays small amounts of rER, mitochondria, and a Golgi apparatus.

Mast cells are not easily identified in human tissue sections unless special fixatives are used to preserve the granules. After glutaraldehyde fixation, mast cell granules can be displayed with basic dyes, such as toluidine blue. It stains the granules intensely and metachromati-

cally because they contain heparin, a highly sulfated proteoglycan (Fig. 5.19).

Several vasoactive and immunoreactive substances are contained in mast cell granules

Mast cells release their granules when appropriately stimulated, as wlTCiilfn Tndividuaris exposed,to an antigen to which he or she has already been sensitized. Sensitization develops after the initial encounter with an antigen. During this first encounter, the immune system recognizes the antigen as "nonself." Immune system cells that express antibody molecules on their surface specific for the antigen Ccognate antibodies) proliferate and differentiate into specialized antibody-secreting cells, plasma cells. These cells then produce antibodies against the antigen. Several major classes of antibodies, called immunoglobulins, are produced. Immunoglobulins of the IgE class are released by the plasma cells and bind to Fc receptors located on the plasma membrane of the mast cells. On subsequent exposure to the same antigen, an antigen-antibody reaction occurs at the mast cell surface that causes the discharge of mast cell granules.

The secretions of mast cell granules can result in immediate hypersensitivity reactions, allergy, and anaphylaxis

Several primary substances found inside mast cell granules are

• Histamine and slow-reacting substance of anaphylaxis (SRS-A), which increase the permeability of small blood vessels, causing edema in the surrounding tissue. In addition, both substances increase mucus production in the bronchial tree and trigger contraction of smooth muscles in the pulmonary airways, causing bronchospasm.

• Eosinophil chemotactic factor (ECF) and neutrophil chemotactic factor (NCF), which attract eosinophils and neutrophils to the site of inflammation. The secretions of eosinophils counteract the effects of the histamine and SRS^KT

• Heparin, a sulfated glycosaminoglycan, which is an anticoagulant. When it unites with antithrombin III, it can table 5.6. Comparison of Features Characteristic of Mast Cells and Basophils

Characteristic Features

Origin

Site of differentiation Cell divisions Life span Size

Shape of nucleus Granules

Surface Fc receptors for IgE antibodies

Mast Cells

Hemopoietic stem cell Connective tissue Yes (occasionally) Weeks to months 20-30 fxm Round

Many, large, metachromatic Present

Basophils

Hemopoietic stem cell

Bone marrow

Days

7-10 fxm

Segmented (usually bilobar) Few, small, basophilic Present

0 0

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