In an allergic reaction, your immune system thinks that a substance, which causes most people no harm, is a threat to your body. Urticaria, also known as nettle rash or hives, takes the form of itchy raised areas, or wheals, on the skin. They are usually pale in the middle and red round the edges and can affect a fifth of us at any one time. Urticaria is the result of touching or eating something to which you are allergic. It can erupt after seconds and normally lasts a few hours.

Angio oedema is a severe reaction in which your face, eyelids and throat swell. It can also hamper your breathing. The outbreaks on the skin resemble those of urticaria, but originate in lower levels of the skin. Though angio oedema can be painful, it does not itch. Contact dermatitis is a condition in which the skin becomes inflamed and develops itchy red patches after contact with a particular substance.

The chief culprits are detergents, some of which may be left in washed clothes, nickel from watch straps, spectacles frames and jewelry, cosmetics, some plants notably primula and poison ivy, chemicals, particularly those in rubber and preservatives in creams and paints. Eczema may be another form of contact dermatitis, but it may also be caused by something you have eaten by stress or by something in the environment.

Neither dermatitis nor eczema is infectious. Around one in 20 people has eczema. What eczema looks and feels like varies from person to person, but most have dry patches of skin that crack, redden and sometimes bleed. You may have small watery blisters.