Allergy is hypersensitive reaction by the body to foreign substances (antigens) which in similar amounts and circumstances are harmless within the bodies of other people. The allergic response develops when the natural immune defence mechanism, responsible for the correct reaction to environmental agents, is disturbed. The allergens are divided into those originating from the natural environment and those from a chemically contaminated environment. The most frequent allergens from the natural environment are inhalant ones present in pollen grains, mould fungi spores and in fragments of mycelial hyphae. The airborne allergens also include: bacteria, house dust mites, epidermis of house pets, allergens of some food products and insect venom. The allergens originating from the natural environment are usually proteins, being high-molecular compounds of molecular weight higher than 10 kDa. Pollen allergens are water-soluble proteins or glycoproteins of molecular masses from 10-70 kDa. Many of them are resistant to pH changes and high temperature, even up to 100 degrees C. Apart from pollen grains, allergens can occur in other parts of plants: roots, stems, leaves, seeds or fruit, in substances excreted by plants, such as juice and volatile oils, or in other bioaerosols of plant origin, e.g. fluids released during treatment of some crops. Proteins of some antigens show some analogies in the amino acids sequence, which determine immunological similarity and cross reactivity. From among factors conducing pollen allergy the most important are genetic and environmental ones (air pollution, exposure to allergens, infections of respiratory tract, diet) and microflora of pollen grains.