As the expected lifespan of man still rises in modern societies, the problems of immunosenescence are getting constantly more and more interesting for scientists. According to the classical theory, cumulating microinjuries within the immune system cause overall immunodeficiency in elderly. However, recently the genetic background is more frequently used for explanation of this phenomenon. One of the main causes of immunosenesence is involution of the thymus. The results of this process are changes in subpopulations and functioning of T cells, as well as mainly secondary dysfunction of other types of immune cells. Although function of some cells (e.g. neutrophils or NK cells) seems to remain unaffected, the immune system in elderly is clearly less effective in comparison with the younger population. Hopefully, studies on this subject will help to find some effective strategies to prevent or to treat this dysfunction.