How antihistamines work 

Antihistamines work by stopping histamine affecting your body's cells in the usual way. They target special molecules called receptors, which are found in your cells.


Histamine is a chemical the immune system uses to help protect the body's cells against infection. The immune system is the body's natural defence against illness and infection.

If the immune system detects a harmful foreign object, such as bacteria or a virus, it will release histamine into nearby cells. The histamine causes small blood vessels to expand and the surrounding skin to swell. This is known as inflammation.

Histamine is usually a useful substance, but if you're having an allergic reaction it's sometimes necessary to block its effects. Allergic reactions occur when your immune system mistakes a harmless substance, such as pollen, for a threat.


Receptors are molecules found in the cell membranes. They react when they come into contact with certain substances.

Antihistamines work by blocking the receptors in each cell, so histamine can't activate the receptors and affect the cell.

Histamine receptors

Histamine receptors cause inflammation and stimulate the production of stomach acid. They're also thought to help stimulate chemicals that transmit information around the brain and may help regulate the immune system.

The majority of antihistamines are designed to block the receptors causing inflammation. Antistamines used to treat stomach ulcers are designed to block the receptors producing stomach acid.

Research is underway to produce an antihistamine which can be used to treat mental health conditions and neurological conditions, such as Alzheimer’s disease and autoimmune conditions, such as rheumatoid arthritis.

Page last reviewed: 24/02/2015

Next review due: 24/02/2017