The immune system normally protects the body from foreign invaders that can cause harm such as viruses and bacteria. During an allergic reaction the immune system turns its defenses against a substance that does not normally cause disease, one that is ignored by the immune system of people without allergies, and this causes allergy symptoms.

When a person with outdoor allergies encounters an allergen such as tree pollen, the immune system attempts to protect the body against the “invading” pollen grain, even though the pollen presents no real threat. The immune system generates a large quantity of a type of defense molecule called an IgE antibody to bind to the pollen. Each IgE antibody is specific to a particular allergen so a person who is allergic to tree pollen but not to grasses would have large quantities of IgE antibodies that recognize tree pollen but none that recognize grass pollen. However, cross-reactivity to allergens is very common, so that being allergic to one type of pollen often puts you a risk for being allergic to other types.

After exposure to an allergen, the IgE antibodies that are produced attach themselves to immune system cells called mast cells. This makes the mast cells primed and ready to be activated. When the person next encounters that allergen, it will bind to the IgE antibodies already attached to primed mast cells, which signals the mast cells to release large amounts of histamine and other chemicals that cause inflammation. It is these chemicals that cause the symptoms of allergies. Outdoor allergens are likely to be encountered in the nose, eyes, or possibly the lungs, leading to symptoms of inflammation in those areas: sneezing and runny nose or congestion, itchy red eyes, coughing, etc.