SRS-A (Slow-reacting substance of anaphylaxis) is?

Slow-reacting substance of anaphylaxis (SRS-A) consists of several leukotrienes, which do not exist in a preformed state but are produced during anaphylactic reactions. This accounts for the slow onset of the effect of SRS-A. Leukotrienes are formed from arachidonic acid by the lipoxygenase pathway and cause increased vascular permeability and smooth muscle contraction. They are the principal mediators in the bronchoconstriction of asthma and are not influenced by antihistamine. Also Know: Eosinophil chemotactic factor of anaphylaxis (ECF-A) is a tetrapeptide that exists preformed in mast cell granules. When released during anaphylaxis, it attracts eosinophils that are prominent in immediate allergic reactions. The role of eosinophils in type I hypersensitivity reactions is unceain, but they do release histaminase and arylsulfatase, which degrade two impoant mediators, histamine and SRS-A, respectively. Eosinophils may therefore reduce the severity of the type I response. Ref: Levinson W. (2012). Chapter 65. Hypersensitivity (Allergy). In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.