Allergens are commonly encountered proteins. In the environment they include tree pollen, grass pollen, weed pollen, animal (cat, dog, horse, rabbit, hamster, gerbil), food proteins and insect venoms. The normal response to these proteins is nothing; that is no reaction is triggered in a person at all.
When an allergic person encounters the same proteins they react with severe treating, itching, sneezing, nasal congestion, or asthma. Encountering food proteins can trigger anaphylaxis in an food-allergic person. The same situation can develop when a venom allergic person is stung by a bee, wasp, or hornet.
These reactions occur because an allergic person’s body produces an antibody of the IgE class– which is specific for any given protein or allergen. Therefore one person might produce IgE to ragweed alone, another to ragweed, cat, aspen trees, and sagebrush..etc. Every combination can and does occur.
If you are highly allergic to environmental proteins you are not guaranteed to have food allergies or venom allergies.