Pollens are tiny protein particles that carry the male gametes (sperm) that combine with female gametes (eggs) in plant fertilization. Pollination occurs with the movement of pollen from the anther of one flower to the stigma of the same flower or another flower.

Plants pollinate at the same time each year. Some plants pollinate in the spring, while others pollinate in the late summer or early fall. Pollination appears to be affected by the relative length of night and day, so geographical location is the key factor. Generally, the farther north, the later in the season a plant will pollinate.

In the United States, trees generally pollinate in February through May, grasses pollinate in May through June, and flowering plants and weeds pollinate throughout the summer and until the first frost.

Pollen seasons in the United States are roughly as follows:

Grass pollen season:

• Southeast and Southwest: March to October
• South: April to September
• Northeast, Northwest, Midwest and Great Plains: May to August

Ragweed pollen season:

• Florida and extreme southern Georgia: July to November
• Deep South: August to November
• Northwest: No season
• Rest of the nation: August to October

Tree pollen season:

• Bottom third of nation: January to June
• Middle third of nation and Northwest: February to June
• Top third of nation: March to June

Weed pollen season:

• Florida, and parts of George, Alabama, Arizona, Nevada and California: April to November
• Northwest and parts of California, Nevada, Idaho, Utah and Arizona: June to October
• Deep South: July to November
• Rest of nation: July to October

The amount of pollen produced and distributed in a given season depends on climate conditions, including wind currents, humidity and rainfall. This is why pollen counts can vary within cities and even during the day. For instance, pollen counts are usually lowest during wet, chilly periods and in the evening hours. They tend to be highest in the mornings, and on warm, dry, windy days.

In the United States, an estimated 20 percent of the population suffers from seasonal allergies, according to the American Academy of Allergy, Asthma and Immunology.

Pollens cause allergies when they lodge inside the mucous membranes that line an individual’s nasal cavities. The white blood cells of the body’s immune system react to this presence by producing the antibody immunoglobulin E (or IgE), which attaches to mast cells in the tissues. When the antibody comes into contact with the pollen (the antigen) again, it triggers a process that results in the release of histamine, leukotrienes and other chemical substances.

These chemicals open the small blood vessels of the nose, causing fluids to leak through the expanded vessel walls and triggering swelling in the nasal passages. The result is nasal congestion and the other symptoms (e.g., watery eyes, itchy nose) of the allergic reaction commonly known as hay fever (medically known as allergic rhinitis).

Not all humans react in the same way to these allergens. For example, a nonallergic person might produce only very small amounts of IgE when confronted by an allergen. In these people, mucus in the airway simply sweeps pollen to the throat, where it is swallowed or coughed out.