Desensitisation or immunotherapy is the subcutaneous injection of the allergic substance (eg: wasp or bee venom, grass pollen, etc.) under the skin. Initially this is undertaken weekly. Extremely small doses of the allergic substance are given initially and this is slowly increased in order to produce a tolerance to the allergic substance. A top dose or maintenance dose is usually reached within 12-15 injections. On this dose allergic reactions to the substance should subside. The protective effect appears to occur very rapidly in the case of bee and wasp venom and also grass pollen, but the benefit appears to get greater with every injection. At the top dose, or maintenance dose, the injections are rapidly lengthened to one every 4-6 weeks. Usually injections are given in alternate arms. An observation period of one hour post-injection is required.

Side effects of immunotherapy
Most people do not notice any problems after the injections. Tiredness in the evening may be noted by some people during the dose-increasing period. Local red reactions, which may be a little tender and occur at the injection site only, may be noted. Although a nuisance, these do not require treatment. If they do, an antihistamine can be taken (eg: Clarityn or Zirtek). The size of this reaction should be reported at the next injection. After an injection you should avoid vigorous exercise or saunas that day. This is because the increased blood supply to the skin will increase the rate of absorption of the injected substance.

Allergic reactions with immunotherapy
Allergic reactions to desensitisation are not common. When they do occur, a blocked nose, sneezing or itching is the commonest complaint. This is because many of the preparations are depot products which release very slowly into the system. When allergic reactions do occur they are usually the result of concurrent viral illness. If you are unwell with an infective or viral illness you should not attend for an injection. Similarly, if you feel extremely tired, the fatigue could be due to an imminent cold or flu. If hay fever or other allergic symptoms are present an injection will not be given. If vaccinations for travel abroad are needed, a week should be left between the two injections. This is because the immune system can only handle one vaccine at a time, and recovery from one should be allowed before a second injection is given.

Contraindications to desensitisation
Patients who are unreliable in their attendance should not be desensitised. Patients taking beta-blockers for blood pressure control or angina must also not be desensitised. This is because the beta-blockers themselves will prevent the action of andrenaline (adrenaline is used to treat patients who suffer an adverse reaction to injection).

Treatment length
It is recommended that 4-6 weekly injections of the top dose or maintenance dose are given for a period of no less than one year. Ideally the injections should be continued for a total of two years and sometimes for longer if improvement is presented but only slowly.