Understanding Hay Fever -- the Basics

Reviewed by Sabrina Felson, MD on May 15, 2019

What Is Hay Fever?

Hay fever is an allergic disorder characterized by an exaggerated immune response to environmental triggers. Common examples include pollen, ragweed, and cats.

Also known as allergic rhinitis, there are two types: seasonal, which occurs only during the time of year in which certain plants pollinate, and perennial, which occurs year-round. 

Typically, if you suffer from hay fever in the spring, you're probably allergic to tree pollens. Grass and weed pollens may be causing your allergic reaction during the summer. In autumn, weeds may plague you, and fungus spores cause problems primarily from late March through November but can be present year round.

People with year-round (perennial) hay fever are usually allergic to one or more allergens found indoors. These include house dust mites, feathers, and animal dander (the tiny skin flakes animals such as cats and dogs shed along with fur), all of which may be found in pillows, and bedding, heavy draperies, upholstery, and carpeting. Another common allergen, mold, is usually found in damp areas such as bathrooms and basements.

What Causes Hay Fever?

If you suffer from hay fever (allergic rhinitis), it's because your immune system views harmless inhaled pollen or other allergens as dangerous substances invading the body. Your system overreacts, flooding your bloodstream with chemicals like histamine and leukotrienes, which inflame the lining of your nasal passages, sinuses, and eyelids and also set in motion other symptoms associated with hay fever, such as sneezing.

All of these symptoms are meant to protect your body either by trapping and expelling the allergen or by swelling body areas, such as the nasal passages, so the allergen can't enter. As a result of congestion in the veins in the lining of your sinuses, dark circles, commonly known as allergic shiners, may appear under your eyes. If you have perennial allergic rhinitis, they may be present all year round. The swelling of your nasal membranes may also close the sinus drainage openings, causing sinusitis. Rhinitis may also be associated with nasal polyps -- small, non-cancerous growths; nosebleeds can also occur during hay fever attacks.

Hay fever is often an inherited trait (genetically determined). The majority of patients with hay fever have a parent or sibling who also has allergies. People with asthma or eczema (allergic dermatitis) are more likely than others to develop hay fever; and about one-third of those with allergic rhinitis also have at least mild, intermittent, allergic asthma.

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