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USMLE – Immunology – Granulocytes


Basophils and mast cells

These two cells have similar functions, but must be considered distinct because of their different developmental histories. Basophils are found in circulation where they comprise about 0.2% of the leukocytes. They are rounded cells about 8-10u in diameter. They have a elongated nucleus usually with two constrictions which is sometimes folded into an S shape.

Mast cells are often elongated. Those from connective tissues seem to be somewhat different from mucosal mast cells. For example, the latter require the presence of Th cells or their products for proliferation whereas connective tissue mast cells do not.

Both mast cells and basophils contain large numbers of cytoplasmic that take up basic dyes. They are so numerous that they obscure other cell organelles. These granules contain large amounts of heparin and eosinophil chemotactic factor of anaphylaxis (ECF-A) and lesser amounts of histamine, serotonin, and precursors of prostaglandins and leukotrienes.

Basophils and mast cells have receptors for both C3a and C5a and for the Fc piece of IgE. Binding of C3a and C5a or cross-linking of membrane bound IgE by allergens induces release 60% – 80% of the granules of both cell types. These factors cause contraction of endothelial cells and vasodilation of capillaries resulting in the redness, warmth and fluid accumulation in tissues characteristic of inflammation. Systemic release (basophils) can cause anaphylaxis. Release of ECF-A attracts eosinophils to the area which seems to be of particular importance in combating parasitic infestations.

Eosinophils make up about 1-3% of leukocytes in circulation. They are similar in appearance to neutrophils except the nucleus is usually bilobed and their granules take up eosin.

The granules are lysosomes in that they contain hydrolytic enzymes, peroxidase, and catalyze. Eosinophils may help limit the inflammation induced by basophils and mast cells in that they contain histaminase.

About half of the material in the granules is major basic protein. They also contain aryl sulfatase. The latter two substances are toxic to parasitic worms (helminthes). Indeed, eosinophils appear to be the major host defense against these invaders.

Lillian Thompson By Lillian Thompson

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