Hay fever is an atopic disease like asthma. This means that the body tends to overreact to certain types of outside matter. One way it overreacts is by releasing histamine, a chemical present in almost every animal cell. Histamine dilates the blood vessels, slowing bloodflow and reducing blood pressure. It can help the body by keeping bloodflow, and therefore infections, localized in a single spot. When it's released in the nasal passages or the airway, however, it constricts them. It can also cause muscle spasms that appear as sneezing and coughing.
Hay fever, like asthma, can be inherited, but particular allergies like grass pollen, ragweed, and so on probably are not inherited. Instead, people inherit the tendency to be allergic. Then, whatever someone is most exposed to will probably become the allergen that bothers them.
These are some of the pollens most likely to cause an allergic reaction:
- Spring - tree pollens such as oak, elm, maple, alder, birch, juniper, olive
- Summer - grass pollens such as Bermuda, timothy, sweet vernal, orchard, Johnson; and weed pollens like Russian thistle and English plantain
- Fall - weed pollens, especially ragweed