Page 966 - Veterinary Immunology, 10th Edition

lung. They contain chondroitin sulfate but little histamine.
VetBooks.ir Connective tissue mast cells, in contrast, arise from fetal liver stem

cells. They are found in the skin, around blood vessels, and in the
peritoneal cavity. They are rich in histamine and heparin. Although

connective tissue mast cell numbers remain relatively constant,
mucosal mast cells proliferate in the presence of intestinal worms.
Connective tissue mast cells are constitutive and T cell
independent, while mucosal mast cells must be induced and are T

cell-dependent. Induced mucosal mast cells disappear within a few
weeks after the parasites are eliminated.

TABLE 29.1
Comparison of the Two Major Types of Mast Cell

Mucosal Mast Cells Connective Tissue Mast Cells
Structure Few, variable-sized granules Many uniform granules
Size 9 to 10 µm diameter 19 to 20 µm diameter
Proteoglycan Chondroitin sulfate Heparin
Histamine 1.3 pg/cell 15 pg/cell
Life span <40 days >6 months
Location Intestinal wall, lung Peritoneal cavity, skin
Many different stimuli trigger mast cell degranulation. The best
recognized of these are specific allergens linked to IgE. Allergies,

however, are but a special type of inflammation. Numerous other
signals can degranulate mast cells, including cytokines,
chemokines, chemical agents, physical stimuli, insect and animal

venoms, and viruses. Many DAMPs, including the defensins,
anaphylatoxins, IL-33, neuropeptides, adenosine, and endothelins
(small peptides from endothelial cells), also trigger mast cell
degranulation.
Mast cells express multiple pattern-recognition receptors,

complement receptors, and the mannose receptor (CD48).
Triggering of their toll-like receptors (TLRs) causes mast cells to
release different mixtures of mediators. Thus bacterial

peptidoglycans acting through TLR2 stimulate histamine release,
whereas lipopolysaccharides acting through TLR4 do not. Bacterial
binding to mast cell TLRs triggers tumor necrosis factor-α (TNF-α)
and IL-6 production. Mast cells can thus use these receptors to
distinguish between different pathogens and release a select

combination of cytokines, chemokines, and other inflammatory

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