Final answer:
After age 20, the immune system begins to decline due to changes in the thymus gland. The thymus gland is responsible for producing and maturing T cells, which are a type of lymphocyte important for the immune response. As people age, the thymus gland starts to shrink, leading to a decrease in the production of new T cells and a decrease in overall immune function.
Step-by-step explanation:
After age 20, the immune system begins to decline due to changes in the thymus gland. The thymus gland is responsible for producing and maturing T cells, which are a type of lymphocyte important for the immune response. As people age, the thymus gland starts to shrink, leading to a decrease in the production of new T cells and a decrease in overall immune function.