When you experience an allergy, it’s often a reaction by the body’s immune system to harmless substances that are seen as “threat.”¹ According to the Centers for Disease Control and Prevention (CDC), one in four people suffer from seasonal allergies.² Although some will grow out of these allergies, many will suffer with them for the rest of their lives.

In the case of seasonal allergies, your body's defenses go into overdrive thinking that pollen, which is usually harmless stuff, is actually a dangerous intruder. The immune reaction releases chemicals, such as histamine, which trigger those annoying allergy symptoms, like sneezing, a runny nose, or watery eyes.

And, because each season ushers in variety of allergens due to climate shifts and air pollution, the presence of airborne pollen and other allergens can lead to allergic reactions all year round. Here are some of the most common causes of allergies by season:

Spring allergies: Tree pollen is predominant, with birch being a common trigger.

Summer seasonal allergies: Grasses such as ryegrass and timothy dominate.

Fall allergies: Ragweed pollen is the main allergen, and it causes severe symptoms of fall allergies.

Seasonal allergies in winter: Indoor allergens such as mold, pet dander, and dust mites are present year round, even when outdoor allergens are dormant, but their numbers tend to increase in winter due to the relative increase of temperatures inside households.

Knowing these triggers can help you get through the seasons with fewer allergic reactions.

Pollen, that pesky airborne allergen, can really put a dent in your warm weather plans if you're allergic. Pollen grains, which are tiny seeds from plants, trees, grasses, and weeds, vary in concentration depending on the season and region. Climate change can extend the pollen season and increase pollen concentrations, exacerbating allergic reactions. Exposure to pollen can trigger hay fever symptoms such as sneezing, a runny nose, and congestion.

Mold spores, the microscopic “seeds” of fungi, thrive in damp environments and can be found both indoors and outdoors. Have you ever noticed a musty smell and felt your allergies flare up? When mold matures, these spores are released into the air and can cause allergies. Mold allergies are most common from July to early fall, but symptoms can occur all year round due to indoor mold growth.

Dust mites, microscopic creatures in your home, are major triggers for year-round allergies and allergic asthma. They thrive in warm, humid environments and feed on human dander found in furniture, carpets, and bedding. Dust mite allergens are the mites’ body parts and feces that cause symptoms such as sneezing and a runny nose.

For a long time, people thought pet allergies were caused by fur or feathers. With the development of allergy research, this theory has been disproven, and we now know what causes pet allergies in the first place.

It turns out that pet allergy symptoms are actually triggered by proteins contained in animal glands, like those found in a cat or dog’s saliva. These proteins may be tiny, but they can pack a huge allergic punch!

For example, in the case of our feline friends, the proteins in cats that can set off your allergies are called Fel d, with Fel d 1 protein (secretoglobin) being considered the most allergenic. These proteins often spread when cats groom themselves and are released when they shed fur.

Things are similar for dogs, as dog allergens, especially Can f 1, are also found in dander, saliva, urine, and serum. It’s possible for specific allergens to vary between dog breeds, but all breeds produce allergenic proteins (even poodles and hairless dogs), so there’s no such thing as a hypoallergenic dog (or cat, for that matter).

Indoor air pollution, which can be five times more concentrated than outdoor air pollution, can pose a significant risk to you if you are prone to allergies. Common indoor pollutants include smoke from cooking and heating, as well as chemicals from cleaning products. These can set off allergic reactions such as headaches and runny noses.