 Serine proteases (tryptase and chymase). Tryptase is selectively concentrated in the secretory granules of human mast cells (but not basophils). It is released by mast cells together with histamine and serves as a marker of mast cell activation. Chymase plays an important role in generating angiotensin II in response to vascular tissue injury. Mast cell chymase also induces apoptosis of vascular smooth muscle cells, particularly in the area of atherosclerotic lesions.
 Eosinophil chemotactic factor (ECF) and neutrophil chemotactic factor (NCF), which attract eosinophils and neutrophils, respectively, to the site of inflammation. The secretions of eosinophils counteract the effects of the histamine and leukotriens.
Newly synthesized mediators include the following:  Leukotriene C (LTC4) is released from the must cell and then cleaved in the ECM, yelding two active leukotrienes— D (LTD4) and E (LTE4). They represent a family of modified lipids conjugated to glutathione (LTC4) or cysteine (LTD4 and LTE4). Leukotrienes are released by mast cells during anaphylaxis (see Folder 6.5 for a description of anaphylaxis). Similar to histamine, leukotrienes trigger prolonged constriction of smooth muscle in the pulmonary airways, causing bronchospasm. This constriction,
TABLE Comparison of Features Characteristic of Mast Cells and Basophils 6.6 Characteristic Features Mast Cells Basophils Origin Hemopoietic stem cell Hemopoietic stem cell Site of differentiation Connective tissue Bone marrow Cell divisions Yes (occasionally) No Cells in circulation No Yes Life span Weeks to months Days Size 20–30 m 7–10 m Shape of nucleus Round Segmented (usually bilobar) Granules Many, large, metachromatic Few, small, basophilic High-affinity surface receptors Present Present for IgE antibodies (Fc RI) Marker of cellular activity Tryptase Not yet established however, cannot be reversed by treatment with antihistaminic agents.
 Tumor necrosis factor  (TNF-) is a major cytokine produced by mast cells. It increases expression of adhesion molecules in endothelial cells and has antitumor effects.
 Several interleukins (IL-4,-3 -5, -6, -8 and -16), growth factors (GM-CSF), and prostaglandin D2 (PGD2) are also released during mast cell activation. These mediators are not stored in granules but are synthesized by the cell and released immediately into the ECM.
Mediators released during mast cell activation, as a result of interactions with allergens, are responsible for variety of symptoms and signs that are characteristic for allergic reactions.
Basophils that develop and differentiate in bone marrow share many features with mast cells.
Basophils are granulocytes that circulate in the bloodstream and represent less than 1% of peripheral white blood cells (leukocytes). Developmentally, they represent a separate lineage from mast cells, despite sharing a common precursor cell in the bone marrow. Basophils develop and mature in the bone marrow and are released to the circulation as mature cells. They have also many other common features with mast cells such as basophilic secretory granules, an ability to secrete similar mediators, and an abundance of high-affinity Fc receptors for IgE antibodies on their cell membrane. They participate in allergic reactions (see Folder 6.5) and together with mast cells release histamine, heparin, heparan sulfate, ECF, NCF, and other mediators of inflammation. In contrast to mast cells, basophils do not produce prostaglandin D2 (PGD2) and interleukin-5 (IL-5). Basophils and their features are discussed in more details in Chapter 10.