Many people may already be suffering from allergy symptoms day to day without realizing it. Do you sneeze, have itchy and runny nose when the season changes? Do you suffer symptoms of itching, skin rash or diarrhoea after eating certain foods? These might be signs of allergy.

Allergy is one of the most common chronic illnesses, and includes allergic rhinitis (nasal allergy), ocular allergy, atopic dermatitis, asthma, food allergy and insect allergy. Things that you commonly come into contact with daily might trigger allergic symptoms, causing significant symptoms.

Allergic reaction occurs when your immune system reacts inappropriately to otherwise harmless substances.

One of the functions of the immune system is to identify substances that are harmful to our health and react accordingly to mitigate the danger. When pathogens such as viruses, bacteria and parasites invade and damage body tissue, danger signals are sent that enable the immune system to identify these pathogens as harmful. After the initial immune response, antibodies and memory cells are formed and re-exposure to the same pathogens subsequently will result in a much more rapid immune response.

On the other hand, when the immune system is exposed to harmless substances such as pollens, dust mites and food, in the absence of danger signals, the immune system identifies these substances as harmless and produces a tolerance response. Antibodies and cells that are capable of blocking or suppressing active immune reactions are produced, which prohibit immune reactions upon subsequent re-exposure to the same substances.

However, when the immune tolerance mechanism breaks down for whatever reason, the immune system then produces active responses to these harmless substances, which result in allergic diseases. Substances that cause allergy are called allergens.

Allergic diseases are often inherited. During the first few months after birth, an infant's immune system is still developing and immunological tolerance still has not fully developed. Some studies showed that in babies who went on to develop allergies, their immune system might take longer than usual to fully mature, thus prolonging the window when allergies might develop.

Some infants with genetic variations that result in skin barrier dysfunction are also at risk of developing allergies. This might be due to increased exposure to allergens through the skin causing sensitization.

Environmental factors present during infancy and early childhood have been found to exert a great influence in the development of allergic diseases. Studies have found that exposure to certain infections during early childhood has a protective effect on the development of allergic diseases. Exposure to certain pathogen products can speed up the maturation of the immune system and strengthen the types of immune responses that protect against allergy. Improvement in hygiene conditions in developed nations during the last two generations might be partly responsible for the rapid increase in the rate of allergic diseases seen in these populations.

The pattern of allergic sensitizations seen in different populations is closely related to the pattern of allergen exposure during early childhood. For example, patients who grew up in temperate climate are more likely to develop pollen allergy, whereas those patients who grew up in tropical climates might develop sensitivity to house dust mite. The pattern of food allergy is also dependent on the dietary habits of the community. For example, peanut allergy is very common in the UK and the USA, but is rare in Israel and Thailand, although peanut is a staple food in the latter two countries. Sesame allergy is the predominant food allergy in the Middle East and Japan, and sesame features prominently in the cuisine of both cultures.

After you become sensitized to an allergen, whenever you are exposed to that allergen, you might develop allergy symptoms. There are two main types of allergic reactions; antibody-mediated or cell-mediated reactions.

Allergic reactions mediated by antibodies are also called Type-I hypersensitivity reaction or immediate-type allergic reaction. When the immune system becomes sensitized to the allergen, a type of antibody called IgE is produced to specifically react with that allergen. These antibodies go through the circulation and end up being trapped on the surface of immune cells called mast cells and basophils. These cells are mainly found in the skin, the respiratory tract and the gastrointestinal tract. When the patient is exposed to the allergen, the IgE recognizes the allergen and triggers the mast cells and basophils to release inflammatory mediators. These mediators can lead to swelling and leakage of blood vessels, itching and airway constriction. This type of reaction occurs very rapidly, usually within minutes of allergen exposure.

Allergic rhinitis, asthma, urticaria (hives), acute food allergy, insect sting allergy and some drug allergy reactions are caused by this type of response.

Cell-mediated allergic reactions are also called Type-IV hypersensitivity reactions or delayed-type allergic reactions. T-lymphocytes, a type of immune cells, become sensitized to an allergen, and upon re-exposure releases inflammatory mediators. This type of reaction typically occurs 24 to 48 hours after allergen exposure. Atopic dermatitis, contact dermatitis, certain food allergy and drug allergy reactions are caused by this type of response.

As different allergic diseases might involve different immune mechanisms, it is important to choose the correct tests to perform based on the medical history and examination.

Since genetics have a strong influence in the development of allergic diseases, children whose parents suffer from allergic rhinitis, asthma, atopic dermatitis or food allergy are at higher risk of developing these diseases themselves.

Parents with allergic diseases	Risk of offspring developing allergies
Mother or father	20-30%
Both parents	50-60%
Both parents with same allergic disease	70% risk of developing same disease

The genetics of allergy is very complicated, and there is no reliable genetic test that can predict the risk of allergic diseases. Studies of umbilical cord blood samples showed that infants who subsequently develop allergic diseases have on average higher levels of IgE in their cord blood than infants who do not develop allergy. Immune cells from the cord blood of the allergic infants also show differences in certain functions.

Using cord blood IgE level and family history, it is possible to identify infants who are at high risk of developing allergic diseases. In future it would be interesting to see if measures taken during infancy and early childhood, such as dietary and environmental modifications, might lower this risk.