Histamines are a group of chemicals that have 3 main functions in the body and these functions are controlled by the cells they are release from and the nature of the receptors that are stimulated with release:

• Modulating immune reactions: released from white blood cells and stimulating H1 receptors to help fight infection but can cause allergic reactions.
• Modulating physiological processes: released from special stomach cells stimulating H2 receptors and cause the release of gastric acid...more.
• Neurotransmitter in the brain released from neurons and stimulating H3 receptors causing serotonin release which has many functions but can cause sedation and figures in to Equine Cushings Disease...more.
• Some metabolic processes of the melanoma cells prevalent in Grey Horses...more.

Perhaps most evident to horse folks are those histamines that are released from some types of white blood cells that help the body fight infection but also cause allergic reactions that are common in horses. This will be the main focus of this article. For more on the other uses of antihistamines refer to the links above for articles on these specific conditions.

Horse folks become aware of histamines when it causes allergic reactions in their horses. Some of the diseases we attribute primarily to the inappropriate release of histamine are:

• Hives (First stages of anaphylaxis, contact hypersensitivity, and possibly food or inhaled hypersensitivity)
• Culicoides Hypersensitivity (Queensland Itch or Sweet Itch)
• Allergic Rhinitis (Sneezing and Head Shaking)

This article deals with the use of antihistamines in allergic reactions, including efficacy, side effects and dosages.

Most histamine in the body is generated in granules in mast cells or in the white blood cells called basophils. Mast cells are especially numerous at sites of potential injury like the nose, mouth, and hooves, internal body surfaces, and blood vessels. When these cells are activated by a antigen they degranulate and release a number of immunologically active chemicals including histamine. Histamine attaches to receptor found on smooth muscle and the endothelium of the airways and blood vessels. The result is vasodilation (dilation of blood vessels), bronchoconstriction (constriction of the air ways), and separation of endothelial cells (responsible for hives), activation of pain and pruritic (itching) nerve endings. Another source of histamine is the venom of bees and wasps.