Allergic reactions to marijuana, while not common, do occur. The symptoms include common allergic symptoms including itching and rash but can also include partial or complete airway constriction.
Yes. Quite frankly, people can be allergic to anything and react very differently from the same thing. It all depends on what your body considers a severe threat to itself.
Yes. As a matter of fact, an allergy to marijuana (THC really) is the reason people get "high". There are a number of people who do not feel any effects from marijuana. Those people are NOT allergic to THC.
Sure. Marijuana is a plant and gives off pollen just like any other plant. If you smoke marijuana the allergy can be even worse because smoke is an allergen as well. Since Marijuana also contains a chemical (THC), a person could have an allergic reaction to that, too. People can be allergic to any natural or manmade substance.