How antihistamines work

Antihistamines work by stopping histamine affecting your body’s cells in the usual way.

They do this by targeting special molecules called receptors, which are found in your cells.


Histamine is a chemical that 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 and it can lead to nearby tissue becoming red and swollen. It can also affect the nerves in the skin, making the skin feel itchy.

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, as a threat.


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

Antihistamines work by blocking the receptor sites in each cell, so that histamine can’t activate the receptors and affect the cell.

Histamine receptors

Four different histamine receptors are found in each cell. They are known as:

  • H1 – the receptor that causes inflammation
  • H2 – the receptor that helps stimulate the production of stomach acids
  • H3 – the receptor that seems to help stimulate chemicals used to transmit information around the brain
  • H4 – a receptor that is currently not well understood, although it may help regulate the immune system

The majority of antihistamines are designed to block the H1 receptor. Antihistamines that are used to treat stomach ulcers are designed to block the H2 receptor.

At present, there are no commercially available antihistamines that can block the H3 or H4 receptors. However, current research is underway to produce such an antihistamine.

It’s thought that H3 blocking antihistamines could be useful in treating mental health conditions, such as depression, as well as neurological conditions, such as Alzheimer’s disease. Recent research also suggests that H3 blocking antihistamines could be useful in helping to relieve neuropathic pain (pain caused by damage or irritation to the nerves).

It’s thought that an H4-blocking antihistamine may be useful in treating autoimmune conditions, such as rheumatoid arthritis, where the immune system attacks healthy tissue.

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