Allergies and / or 'hypersensitivities' are overreactions of the immune system. These overreactions are particular to substances in some people. There are four groups of Hypersensitivities. The classifications are based on what parts of the immune system are activated, as well as how long it takes for a reaction to occur. In type I hypersensitivity, a particular foreign substance, 'allergen', and reacts with a specific type of antibody called immunoglobulin E – this reaction causes symptoms that will appear within minutes. In type IV hypersensitivity, the allergen reacts with a specialized type of cell called a T-lymphocyte. The reaction time of type IV is quite long it hours or days until any symptoms appear.

Type I allergic reactions can be inconsistent in severity, it may appear different from time to time, causing a rash the first time and anaphylaxis the next. Type I allergies can be related to just about anything: foods, plants, insect venoms, animal dander, dust mites, mold spores, occupational substances (for example latex), as well as drugs and penicillin. There can also be cross-reactions, where someone allergic to grass pollen, for instance, may also react to melons and tomatoes. The most common food-related causes of severe anaphylactic reactions are peanuts, tree nuts, and shellfish.

Type IV delayed hypersensitivity reactions are most often skin reactions. Common examples include reactions to metal and jewelry. They occur when an allergen interactions with specific T lymphocytes. Type IV hypersensitivity is usually a reaction (redness, swelling, hardening of the skin, rash, dermatitis) observed at the exposure site hours to days after exposure.

Type I acute allergic reaction can cause a rash, dermatitis, and itching, while in the long term the allergy may cause atopic dermatitis and eczema. In the respiratory tract, the acute reaction causes coughing, nasal congestion, sneezing, throat tightness. This can in long term led to asthma. It can also cause red itchy eyes. Acute allergic reactions in the gastrointestinal system start in the mouth with swelling of the tongue and throat, followed by abdominal pain, muscle spasms, vomiting and diarrhea.

Any severe acute allergic reaction can be potentially life threatening, causing anaphylaxis, a reaction spread through the entire body that can start with agitation, a 'feeling of impending doom' pale skin (and due to low blood pressure), and / or loss of consciousness (fainting). Anaphylaxis can be fatal without the rapid administration of an adrenaline or cortisone injection.

The best treatment for allergy is to be cautious and keep away from the substances that trigger your allergy. Allergies are unpredictable and may go away after a couple of years but they might as well stay with you forever.

If you fear an allergy it is of high importance to take a test and determine what substances you are hypersensitive to.