Antihistamines are a class of drugs primarily used to treat the common cold, the flu, allergic rhinitis and other allergies. They are usually taken as an inexpensive generic drug purchased over-the-counter without a prescription. Many are available on prescription only to treat more severe allergic reactions and as a sedative and sometimes for insomnia, motion sickness and vertigo as well as gastrointestinal disorders.
Histamine:
Histamine is a local acting vasoactive chemical that is present in almost all body tissues and has a wide range of effects: vascular (dilates small blood vessels and increase capillary permeability); respiratory (bronchial constriction); gastrointestinal (increased secretion of acid); CNS (neurotransmitter).
It is synthesised and stored in mast cells and basophils and release from these cells is stimulated by allergic and non-allergic mechanisms:
1) Allergic: Increased production of IgE antibodies in response to specific antigens bind to the surface of mast cells and basophils. On re-exposure to antigen, it is bound to the antibody leading to mobilisation of intracellular calcium. This causes the histamine storage granules to fuse with cell membrane and release their contents.
2) Non-allergic: Some drugs act directly on mast cells causing them to release histamine; cell injury also releases histamine
Two main types of histamine receptor:
1) H1 – involved in vasodilation, increase capillary permeability, bronchoconstriction, itching and pain from direct stimulation of sensory nerves, sedation (from action as a neurotransmitter)
2) H2 – involved in increase secretion of gastric acid
Allergic reactions are mediated by histamine increasing vascular permeability allowing fluid to escape from capillaries into tissues leading to the classic symptoms of an allergic reaction (eg runny nose and watery eyes).
Antihistamines:
Antihistamines block the effects of histamine and are used to reduce symptoms due to type 1 allergic diseases. They can be classified according to the histamine receptor that they act upon. The two main classes of antihistamines are H1-antihistamines and H2-antihistamines.
Other antihistamines also target the H3 receptors and H4 receptors.
The Federal Drug Administration (USA) has approved the following indications for antihistamines:
H1 Antihistamines: allergic rhinitis; allergic conjunctivitis; allergic dermatological reactions; sinusitis; urticaria; angioedema; atopic dermatitis; bronchitis; motion sickness; nausea; vomiting
H-2 Antihistamines: peptic ulcer; acid reflux; gastritis
See:
H1 Antagonists/’Anti-histamines’
H2 Antagonists
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