Hay fever, so called pollen allergy, is basically the irritation and inflammation of the mucous membranes in the nose. This is usually caused by pollens, in spring, fall or summer. These pollens (the attackers) are in the air traveling. So one preventive measure towards hay fever is to stay indoors. Somehow, And plants that are pollinated by animals or insects usually have pollen that is too heavy to travel by air.
Allergy can come from cat hair, peanuts, dust and much more. And the allergic response to these is more likely similar to allergic response to pollen. How does pollen cause allergy to a person? When a pollen grain first enters the body, it will have contact with the antigen presenting cell. This antigen presenting cell then engulfs the pollen and pushes little bit to its own surface. This work of the antigen presenting cell is helpful because is allows visibility to other immune cells (in particular to the T-cell). to detect this invader since other immune cells cannot detect foreign invaders. The T-cell in turn relays the information onto even more immune cells. Among cells that will receive the information, the B-cell, of which upon hearing the invaders, will start producing large amounts of antibodies called immunoglobulin E.
Thanks to antibodies. They swim around the blood and attach themselves to more immune cells called mast cells and basophils. So when the allergen enters the body, the antibodies will allow them stick onto these cells. And when this happens, the mast cells and the basophils undergo a process called degranulation where they release large amounts of chemical compounds. These little molecules trigger the dilation of vessels in the nose and heavy mucous secretion resulting in a runny nose sneezing and congestion.