March 20, 2012
Sometimes your body can have a bad reaction to a normally harmless substance. This is called an allergy. It’s caused by a glitch in your body’s immune system.
Normally, your immune system protects you from disease. Special cells recognise germs and viruses, attacking and killing them. Your immune system learns to recognise diseases by exposure. Sometimes, your body can confuse a harmless substance with a disease agent and begins an immune response, resulting in an allergy. This process is called sensitisation. The substance which triggers the allergy is called an allergen.
When your body is exposed to an allergen, it releases a substance called histamine from special cells known as mast cells. There are mast cells in your skin, your lungs, the membranes inside your nose and in your intestines. Histamine causes tissues to swell and become inflamed, causing the distressing symptoms of allergies. These can include itching, coughing and sneezing in the case of allergies affecting the nose, throat and lungs, and nausea, tummy aches or diarrhea in the case of allergies affecting the intestines. Rarely, in very severe cases, allergies can be life threatening. They can cause the airways to close up, preventing the patient from breathing.
To become allergic to a substance, you first need to be exposed to it. You may not become allergic straight away — it can take many repeated exposures to become sensitised to a substance. Some people only become allergic to a substance after years of repeated exposure. If you’re in contact with a substance while suffering from an allergic reaction to a different allergen, you may become allergic to the new substance as well. An example would be a person who uses a particular soap while suffering from dermatitis and becomes allergic to the perfumes used in that soap.
Common allergens include house dust (in fact, it’s the debris created by house dust mites that you are allergic to), animal dander (skin cells shed by pets), feathers and pollen from flowering plants. Notorious plant culprits include goldenrod and ragweed, as well as some tree pollens, although not everyone with a pollen allergy is particularly sensitive to these. If you have a skin allergy, you may be sensitive to common fragrances; many people become sensitised to limonene, which is used to give washing-up liquid its lemony smell, through repeated exposure while washing the dishes. You can also become allergic to oils and other constituents of soaps, shampoos and cosmetics; some people are sensitive to peanut oil or lanolin, for instance.
Allergic reactions to hair dyes are very common. You may have used a hair dye with no problems in the past, only to experience an allergic reaction to it after many uses because you have become sensitised to that dye. That’s why you should always do a patch test before you dye your hair.
Food allergies are increasingly common, for reasons that are not well understood. Peanuts, for instance, can be particularly dangerous, causing life-threatening allergic reactions in sensitive people.