Final answer:
The gland that is large in children and reduces to a small remnant in adults is the thymus gland, which is essential for immune system development during early life.
Step-by-step explanation:
The thymus is an organ of the immune system that is larger and more active during infancy and childhood, and begins to shrink into a small remnant of fat and fibrous tissue in adults. Its endocrine function is the production of a group of hormones called thymosin that contribute to the development and differentiation of T lymphocytes, which are immune cells.
The thymus gland contributes to the immune response and is involved in the development and maturation of T-cells, which are important for the body's defense against infections and diseases. As we age, the thymus gradually atrophies, becoming a small remnant of fat and fibrous tissue in adults.