This paper presents the results of the studies of biological behavior and morphology of the lymphoid system in gnotobionts–animals with controlled microflora, namely: 1) age-related changes are determined in the thymus – the central organ of the lymphoid system in gnotobionts, in which maturation of the T-dependent lymphocytes is delayed, causing inhibition of the cellular immunity reactions; 2) comparative cytological profile and morphology of the peripheral lymphoid organs (spleen, visceral and somatic lymph nodes) are analyzed with regard to age; 3) the leading morphofunctional mechanisms responsible for development of non-coronary damages of the myocardium and the microcirculatory bed are indicated; 4) test morphological studies of the mucous membrane of the ileum are performed with a detailed cytological profile of the lymphoid tissue followed by determination of presence or absence of the structures of both “acceptor” and “protective” immunity.