An extract of the allergen is injected under the skin in gradually increasing doses. The results are most satisfactory when the injections are continued throughout the year.

The efficacy of the treatment is variable. Some people report marked improvement in their symptoms and others see little effect.

The major problem with desensitisation treatment is that some people react very badly to the injections. Reaction can vary from mild cough and sneezing to full anaphylactic shock — which can be fatal.

Desensitisation is not widely used in South Africa or Britain, but has remained popular in the USA. It must only be attempted by a skilled allergy specialist, and full resuscitation facilities should be available.