Here is a nice Cannabis Culture article about marijuana allergy and what can be done about it.
If you aren't allergic to THC, but to other substances in grass, you could try using very refined preparations that contain only cannabinoids. These include well-rinsed water hash, water-washed kief or ether-refined THC oil. While not totally free of the allergens, these preparations contain very small amounts of plant matter. You may also be able to vaporize marijuana, since this turns the cannabinoids into a vapor for inhalation without combusting the plant material. It is more pure than smoking. Many vaporizers are available on the market.
To test for an allergy yourself, you can try rubbing a piece of fresh bud on a small area of skin on your arm and see if a rash develops. If it does, there's a good chance this is the case.