An organ that stops working in childhood But the child just needs it☝🏻 It's the thymus. An organ of the immune system where the maturation, differentiation and immunological "training" of T-lymphocytes takes place. The thymus is essential for creating "initial" immunity, as its function begins to be inhibited at puberty. The body itself deprives it of blood supply and the functional thymus tissue is replaced by fatty one. The thymus almost completely disappears (decreases even more in size) in elderly people. The reason hides in a decrease in immunity in the elderly.