An infected plant is a quantitatively distinct system. Its specific character is due to the nature of host plant and the pathogen, as well as to the character of the mutual effect induced by them. Mutual induction includes the biosynthesis of organoids (mitochondrias, ribosomes, etc.), isoenzymes, and interferon-type proteins and others. The leading role in the system of the defence mechanisms of a plant is played by the protein components of the protoplast, the molecular organization of the biostructures and the energy metabolism regulating systems. The nature of the changes is specific. It reflects the mutual adaptability of the partners formed in the course of their mutual biological evolution (N.I. Vavilov).