Sinus problems, hay fever, bronchial asthma, hives, eczema, contact dermatitis, food allergies, and reactions to drugs are all allergic reactions associated with the release of histamine and other autocoids, such as serotonin, leukotrienes, and prostaglandins. Histamine release is frequently associated with various inflammatory states and may be increased in urticarial reactions, mas-tocytosis, and basophilia. Histamine also acts as a neu-rotransmitter in the central nervous system (CNS). Upon release from its storage sites, histamine exerts effects ranging from mild irritation and itching to ana-phylactic shock and eventual death.
Histamine is found in animal tissues and venoms and in many bacteria and plants.Within the human body, the largest histamine concentrations are in the skin, lungs, and gastrointestinal mucosa, while concentrations are smaller in almost all other organs and tissues. Histamine is present in human plasma at relatively low concentrations (usually less than 0.5 ng/mL); in contrast, whole-blood levels can be as high as 30-fold greater. Substantial quantities of histamine are present in urine, with excretion rates varying from 10 to 40 ^g per 24 hours.
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