Thymalin in developing respiratory organs of human fetus.

We studied the appearance of immunomodulator thymalin in human respiratory organs during early embryogenesis. Thymalin accumulated in young cells of airway epithelium. In the alveolar part thymalin-positive cells were diffusely spread. Mature T cells (CD3+) and the main regulatory elements (CD4+ and CD8+) were detected during the same period in the lungs in the absence of thymic microenvironment. The function of immune elements forming in fetal lungs is local protection of the fetus from potentially aggressive maternal cells and infectious agents entering the body through the trachea and fetal blood vessels.