A definition of allergy is proposed, including its two stages: 1) hypersensitivity (without clinical manifestations) and 2) hyperreactivity with clinical signs. A specific immunologically mediated allergy depends on the presence of antibodies of different isotypes and immune T and B lymphocytes against an allergen. Nonspecific allergy is hypersensitivity and hyperreactivity of the immune system and any cells, as well as humoral systems, to the pathogen. Anergy is the second type of pathologically altered reduced reactivity that can be of allergen-specific (immune) and non-immune types.