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Why does T-cell production decrease as people grow older?
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lymphatic system
The human lymphatic system, showing the lymphatic vessels and lymphoid organs.
The production of T cells decreases as people age primarily due to the natural involution, or shrinking, of the thymus.
While the thymus reaches its greatest size relative to the body during fetal life and early childhood, it begins a slow, lifelong process of shrinking at the onset of puberty. During this involution process, the thymic cortex—the specific site where T cells proliferate and differentiate—becomes thinner. As the functional tissue diminishes, the lymphocytes are gradually replaced by fat tissue. This structural change is a normal aspect of aging and contributes to the overall decline in the efficiency of the immune system over time.123