|MadSci Network: Immunology|
The short answer to your question is that no, you cannot be allergic to something without having been exposed to it before. Allergic responses are due to the the production of a specific type of antibody called IgE which bind to immune cells in your body and cause the release of a number of factors that drive the inflammatory process we call an allergic reaction. What causes that production of IgE against the "allergen" is still unknown. There are animal models which suggest that it is the type of T cell response elicited that results in the allergic response. I would, however, take this data with a grain of salt. The animal models (mostly mice) require preimmunization with the allergen plus a particular immune stimulant (aluminum hydroxide) to create the allergic response. While this does lead to good information about what happens during an immune response, it tells us nothing about how people become allergic to something. The tendency to develop allergies can run in families, suggesting that in part there is a genetic component. The rise in allergies and asthma in developed nations (the U.S.A and European nations) has prompted several suggestions, the one with the most credence being the increase in pollutants in the air. It is likely that several things in combination result in allergies, and this is why we all seem to have our own unique pattern of allergies.
Try the links in the MadSci Library for more information on Immunology.