Final answer:
The process of the human thymus degenerating after birth is known as involution, characterized by the replacement of thymocytes with fat tissue. This biological change is genetically programmed and has significant implications for the immune system, particularly as it relates to aging and potential therapies for immunosenescence.
Step-by-step explanation:
The human thymus begins to degenerate as early as one year after birth. This process is called involution and is marked by the accumulation of fat once occupied by thymocytes. Hence, the correct answer to the question is 'b. involution; fat'.
Thymic involution is a well-documented phenomenon observed not just in humans, but across all vertebrate species that have a thymus gland. Research in animal models, such as mice, has indicated that this process is likely genetically programmed. A key gene involved in the maintenance of the thymic epithelial cells, crucial for the development of naive T cells, is FOXN1, which sees diminished expression with age. Consequently, the thymus shrinks and is gradually replaced by fatty tissue, reducing its capacity to produce new T cells.
Understanding this process is of great importance, as it has implications on the functioning of the immune system throughout life, potentially opening doors to investigate therapies to delay or reverse immunosenescence—the gradual deterioration of the immune system associated with aging.