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To understand fundamental defense mechanisms of an individual and to perceive organization and functioning of the human immune system, it is inevitable to sum up its phylogenic development. Lymphoid organs are crucial in regulation of physiological lymphocytes development and subsequently play a significant role in appropriate immunological responses to foreign pathogens. Throughout the evolutionary tree, the primary lymphoid organs have emerged earlier than the secondary lymphoid organs. Considering the sites for cell populations’ development responsible for adaptive immunity, B lymphocytes differentiation and maturation have considerably differed during phylogeny as well as ontogeny. On the contrary, T lymphocytes development is defined exclusively in the thymus. From the evolutionary point of view, location of primary lymphoid organs must have been sophistically pre-programmed in terms of their function. Need for thymus evolving from the foregut supports the fact of emerging diverse repertoire of antigen receptors. The thymus represents the very first lymphoid organ evolved in Vertebrata to deal with potentially autoreactive, somatically heterogeneous T lymphocyte receptors. The necessity of maintaining an immunological integrity was the most crucial stimulus for evolution. Thymus as a primary lymphoid organ constitutes an eminent structure that markedly differentiates the higher Vertebrata from the rest of the animal phyla. The present paper is meant to provide a profound evolutionary insight into the lymphoid organs with the emphasis on the thymus morphology through the phylogenesis.

Morphological view on the evolution of the immunity and lymphoid organs of vertebrates, focused on thymus