The major eicosanoid product of mast cells released in large quantities during allergic and asthmatic anaphylaxis; also produced in the brain via an alternative pathway involving a soluble, secreted PGD-synthase also known as β-trace where it induces normal physiological sleep and lowering of body temperature; inhibits platelet aggregation, relaxes vascular smooth muscle, and inhibits human ovarian tumor cell proliferation (IC50 = 6.8 µM).