Your immune system protects you against all kinds of antigens -- unwanted, invasive agents like viruses, bacteria and other harmful substances. When an antigen is detected, the immune system sends out immunoglobulins (you may know them as antibodies) to get rid of the intruder.
There are four different types of immunoglobulin that your body can produce in response to the presence of a harmful substance: G, M, A or D. These immunoglobulins attach to other immune cells called mast cells. When this mast cell/immunoglobulin hybrid locates the antigen, it sticks to it. Now, the immune system has the antigen right where it wants it. The mast cell then releases histamines and other chemicals to destroy the immobilized antigen.
Like all great complicated systems, sometimes the immune system gets a bit wonky. It's possible for a perfectly harmless substance to be mistaken for a harmful agent, triggering an abnormal response in the immune system. This is an allergic reaction. The substance could be dust, cat hair, pollen, peanuts or any number of things. But when your immune system detects it, it promptly freaks out and releases a fifth kind of immunoglobulin: E, also known as IgE (immunoglobulin E). People who have no allergies don't have IgE released into their bodies. It's the presence of IgE that determines whether or not an allergic reaction is taking place.
We've already mentioned common allergic symptoms like skin rash, runny nose and watery eyes, but it can get much worse than that. Some people have allergic reactions that result in anaphylaxis, which is something like a nuclear allergic meltdown that can be life-threatening. Anaphylaxis can cause severe reactions to many parts of the body at once, including the throat, lungs and skin.
Fortunately, anaphylaxis can be treated with epinephrine (adrenaline) which helps end the state of shock by boosting blood pressure and opening up the airways in the lungs. Epinephrine can be administered (or self-administered) via injection into the thigh with a small, penlike device that many allergic children and their parents carry with them at all times.
Now that we know how allergic reactions happen, let's find out how you diagnose an allergy.