From the beginning of life, human immune system is affected by interactions with different microorganisms. Immediately after the birth bacteria colonize mucous tissues and namely the gastrointestinal tract, which was antenatally sterile. Bacteria are involved in allergic inflammation by two different ways. They can either initiate the allergic response or they can diminish it. To the negative factors, which can trigger the allergic response belong superantigens, namely the bacterial enterotoxins. Positive factors include mycobacterial antigens, intestinal microflora, pathogen-associated molecular patterns and CpG motives that belong to components of the bacterial genome DNA. The effect of lipopolysaccharide from gram-negative bacteria on the development of allergic inflammation is two faced. Lipopolysaccharide has a protective role when the infection precedes the contact with allergen for a long time. Short period between the infection and allergen exposition leads in a sensitized person to the provocation of allergic disease. These and other factors are important in relation to the so-called hygiene hypothesis where the T regulatory lymphocytes seem to play the key role in the balance between Th1 and Th2 lymphocytes.

        Key words: allergy, bacteria, asthma, atopic dermatitis.