Illustration showing common allergens

Measured responses

The immune system can overreact

The immune system does not always behave as we would like it to. Autoimmune disease occurs when the immune system has trouble telling self from non-self. In rheumatoid arthritis, for example, immune cells attack cells in the patient’s own joints, causing pain and inflammation.

Allergies are caused by overreactions to foreign proteins known as allergens. For example, in hay fever, the immune system reacts to wind-blown pollens from grasses, weeds and trees, and makes antibodies against them. Immune cells called mast cells have antibodies attached to their surface, and when these bind to their specific allergens they trigger an immune response.

This response includes the release of chemicals, including histamine, which are responsible for many of the symptoms of allergy. Histamine causes inflammation, drawing fluid into the affected zone and, in the case of hay fever, out of the nose. Hence the snotty mess and the antihistamine tablets. Meanwhile, in asthma, which is mainly thought of as an inflammatory disease, these chemicals may also make the muscles in the airways contract, causing breathlessness and wheezing.

Anaphylaxis is a severe and potentially fatal allergic reaction that affects the whole body, often just minutes after exposure to an allergen. It can be caused by many things, including peanuts, shellfish, eggs, bee or wasp stings, and drugs. Anaphylaxis can be fatal. People at risk often carry pen-like injectors that contain adrenaline to use in an emergency.

Lead image:

Illustration © Glen McBeth


About this resource

This resource was first published in ‘Immune System’ in January 2015.

Microbiology, Immunology, Medicine
Immune System
Education levels:
16–19, Continuing professional development