[Betarulz!]

First off, you must understand that Lactose is a disacchride (composed of two sugar molecules: glucose and galactose). In order to break the bond between these two monosacchrides there is an enzyme that specifically breaks the bond between them. This enzyme is called lactase. Surprisingly, this enzyme is present in gradually lower levels as you become an adult.

Lacking lactase or having some sort of mutation that lowers the activity of the enzyme is what being lactose intolerant is. Without that enzyme your body resorts to other methods of breaking the lactose down, or it passes through to your intestine where the bacteria break it down into food for themselves. They will produce some sort of chemical intermediate that causes the intestinal pain and diarrhea associated with lactose intolerance.

Basically all allergies are the same. Exposure to anything can create proteins known as IgE proteins that will bind to Mast cells that tend to be concentrated in the areas you associate with allergic reactions. Upon repeated exposure to these substances they interact with the specific IgE proteins on the mast cells. This causes the mast cells to release histamine (a hormone) which causes the reactions we are familiar with into the surrounding tissue. Depending on which sort of tissue the IgE coated mast cells are located causes the varying types of allergic reactions: ie skin = hives, sinuses = runny nose, eyes = watery eyes. This is why differnet peopel react to the same substance in different ways and why the same person will react to different things in different ways.

The interesting thing, is that while being likely to have allergies is genetic, you need to have exposure to the substance to become allergic to it. That's why a lot of people claim to have "resistance" to poison ivy and such, but the more they handle it, the more likely they are to have a reaction to it.

If you know you have never been exposed to something that you are currently allergic to, it simply means you were likely exposed to some substance that has an analogous chemical structure.