Allergies to pollen involve an interaction between abnormal human immune systems and allergenic plants and fungi. Tiny airborne pollen grains and fungal spores work their way into people's respiratory systems. Allergic reactions occur when some immune systems mistake these substances for harmful invaders, such as bacteria or viruses.
Attempting to neutralize the danger, the body releases histamine and leukotriene chemicals, which produce inflammatory side effects that patients experience as allergy symptoms. The Cleveland Clinic relates that air-pollinated plants are more likely to cause hay fever than insect-pollinated flowers.
Ragweed
Highly allergenic ragweed and related plants cause three-quarters of all hay fever conditions in the United States, as the Asthma and Allergy Foundation of America (AAFA) reports. Ragweed, mugwort and similar weeds reproduce in late summer and early fall, discharging a billion pollen grains per plant.
Humans ingest these ultralight granules that travel on air currents when they stick to the mucous membranes of the eyes and nose. People who suffer allergic reactions---about one-fifth of all Americans---develop symptoms when their bodies resist the allergens circulating through their bloodstreams.
Trees
Like ragweed, trees generate prodigious amounts of pollen in bursts, causing extreme springtime allergy symptoms in many hay fever patients. According to the National Institutes of Health (NIH), other common allergenic trees include cottonwood, hickory, walnut, alder, birch and juniper.
Some patients can experience allergic reactions to pollens whose sources grow in distant regions. The height and spacing of certain trees, such as the mountain cedar of the South Central United States, allows their pollen to travel hundreds of miles.
Grasses
Summertime grass pollination occurs in waves across many species. Hay fever patients may suffer extended allergic reactions to several types, including Bermuda grass, orchard grass, Johnson grass, timothy grass and blue fescue.
While any level of exposure to grass allergens will affect sensitized patients, symptoms will compound after increased contact with pollinating grasses, such as walking in large fields or mowing a tall lawn. The NIH notes that grass allergy symptoms may also intensify on days that are hot, dry and windy.
Fungi
Leaf molds are allergenic fungi that grow on decaying plant matter and use air-transported spores to reproduce. People can inhale the spores that they disturb by walking through leaves or that become airborne on wind currents.
The AAFA reports that fungal reproduction can cause allergy symptoms any time there are downed leaves. High mold counts most commonly occur in late summer, but can persist all year in some areas.
