In about 90 per cent of cases there is no apparent cause. Sometimes, however, it is possible to identify the cause of the nettle rash and it appears to be most commonly due to an allergic reaction. Any of the following may trigger symptoms:

a particular kind of food that causes an allergy. Eggs, nuts, strawberries and seafood are some of the common culprits.
contact with pollen or fungal spores.
medicines, most commonly, non-steroidal anti-inflammatory drugs (NSAIDs), antibiotics, and ACE-inhibitors.
insect bites.
infections, which may be caused by viruses, parasites or bacteria.
contact with plants such as stinging nettles.
contact with animals or creatures, such as stinging jellyfish.
physical stimuli, such as feeling cold or warm, sunlight, water, sweating or pressure.
substances, such as perfumes, preservatives, colourings, nickel or tar.

During an allergic reaction, a chemical called histamine is released by cells in the skin. Histamine makes the blood vessels dilate and become more permeable (leaky) so that fluid seeps out into the skin.

The rash itself is due to a local build up of an excessive amount of fluid in the skin (oedema). In severe cases this may be followed by swelling in other parts of the body as well.

If swelling occurs around the eyes, lips, tongue, larynx or hands it is called angioneurotic oedema.