Explain the process by which an attack of hay fever follows exposure to pollen

Hay fever is a manifestation of allergic reaction from pollen, which is prevalent during haying season. Hay fever is commonly known as allergic rhinitis caused by an oversensitive immune system, particularly the lymphocytes, leading to a misdirected immune response. The body undergo a sensitization when one develops IgE antibodies and develop a strong TH2 response against pollen. Then, this newly develop IgE antibodies sticks to basophils and mast cells with no signs and symptoms yet. However, when one is expose again to pollens, mast cells will be activated and release mediators who are responsible for immediate vascular and smooth muscle reaction. After this, local clinical manifestation of hay fever will be observed. 

genetic variation will increase because of a new habitat and food source.

explanation: