Molecular model of histamine, a chemical involved in an allergic reaction. The atoms making up histamine are carbon (green), nitrogen (blue) and hydrogen (white). Histamine is associated with the mast cells found in connective tissue. It causes dilation and increased permeability of small blood vessels. It is released in large quantities after damage to the skin, producing a characteristic red rash and itching. Histamine also gives rise to the symptoms of allergic conditions like asthma.