Histamine. Polarised light micrograph of crystals of the chemical histamine. Histamine is present in nearly all body cells. It is derived from the amino acid histidine, and is mainly manufactured in mast cells found in connective tissue. After damage to the skin (such as in an insect bite) the mast cells release large quantities of histamine. This causes dilation and increased permeability of small blood vessels. This produces inflammation characteristics such as a red wheal and itching. Histamine also gives rise to the symptoms of a number of allergic conditions, including asthma. Magnification: x50 at 4x5 inch size.