The allergens triggering the disease are either protein or protein-carbohydrate structures, which can be of different origin ranging from the mite in our residential environment, pollen grains, and technological materials with varying molecule weight of certain professions.

Most frequently the pollens of different trees, grasses, flowers and weeds are responsible for the forming of the so-called seasonal rhinitis, which, depending on the blooming of plants, can last from a few weeks up to months. Wind-dusted plants producing pollens of low specific gravity in large quantities induce allergic symptoms in a remarkable number of allergy patients. Pollens of wind-dusted plants are 15-20 µm large. These pollen grains can get to distant places in a dry, windy weather, thus triggering mass morbidity far from their original location as well.

There are three main periods of the pollen season in Europe:

1. The spring season ("tree-season": pl. hazel, birch, alder, ash) covers the time when pollen grains are produced and appear from 1st February to 30th April. The initial date of blooming varies, but it is mostly the beginning of March.
2. The early summer season ("grass-season": e.g. phleum phleoides and pooideae, festuca, cocksfoot, some cereals) includes the period between 1st May and 31 July, which is mostly characterized by herbaceous plants (wild grasses, cereals, early summer weeds) and the pollen of some trees.
3. The late summer-autumn season ("weed-season": e.g. artemisia, rip grass, ragweed, nettle etc.), meaning pollen grains produces after 16 July are dominantly characterized by the pollen of herbaceous weeds.

Between certain food and pollen types cross-reactions may occur: water melon and banana may trigger allergic symptoms in patients allergic to ragweed, in the case of patients allergic to mugwort celery and some spices can do the same, while apple, cherry, hazel and peach can cause symptoms to those suffering from birch.