Why do people have 'allergies'.What causes it?

Allergies are caused from allergens (things that are you are allergic to) entering the body, and the body identifying it as foreign. What makes is classified as an allergic reaction is the body has what's called a hyper-reaction. It overcompensates to eliminate the foreign substance. Everyone is different in what they are allergic to and what their bodies respond to and to what level. Some people don't have any known allergies, or a few, other are allergic to everything.

It caused by an over ambitious immune system. ~
Often caused by under 1 year olds getting antibiotics, which kill off germs the immune system normally uses to sharpen its teeth on. So, it starts being overzealous with the host body.
causes :

food-eg peanut, rice (protein), chocolate ...
environment-flower .
whether-cold ...
my anti bodies are like the Rebel Forces, anything that comes in gets attacked..excessively. As a result....all shellfish is off-limits, all pets...cats...dogs...birds etc....grasses, spores...molds...yada..yada......

good thing is my house is clean :-)
An allergy is an overreaction of the immune system to a substance that's harmless to most people. But in someone with an allergy, the body's immune system treats the substance (called an allergen) as an invader and reacts inappropriately, resulting in symptoms that can be anywhere from annoying to possibly harmful to the person.

In an attempt to protect the body, the immune system of the allergic person produces antibodies called immunoglobulin E (IgE). Those antibodies then cause mast cells (allergy cells in the body) to release chemicals, including histamine, into the bloodstream to defend against the allergen "invader."

It's the release of these chemicals that causes allergic reactions, affecting a person's eyes, nose, throat, lungs, skin, or gastrointestinal tract as the body attempts to rid itself of the invading allergen. Future exposure to that same allergen (things like nuts or pollen that you can be allergic to) will trigger this allergic response again. This means every time the person eats that particular food or is exposed to that particular allergen, he or she will have an allergic reaction.

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The tendency to develop allergies is often hereditary, which means it can be passed down through your genes. However, just because you, your partner, or one of your children might have allergies doesn't mean that all of your kids will definitely get them, too. And someone usually doesn't inherit a particular allergy, just the likelihood of having allergies.

But a few kids have allergies even if no family member is allergic. And child who is allergic to one substance is likely to be allergic to others as well.
http://www.kidshealth.org/parent/medical...
I have learned from reading Bragg's book on Apple Cider Vinegar that our body is about 80 percent alkaline from eating alkaline forming foods and about 20 acid from eating acid forming foods. But when we get off balanced by eating to much acid forming foods then allergies start to set in. And speaking from experience it is 100% true. Good Luck!