The primary function of the thymus gland in the immune system is to facilitate the development and maturation of T lymphocytes, which are crucial for adaptive immunity. Located in the upper chest, the thymus provides an environment for the differentiation of precursor cells into functional T cells. This process includes positive and negative selection, ensuring that T cells can effectively recognize foreign antigens while remaining tolerant to self-antigens, thus preventing autoimmune responses. A properly functioning thymus is essential for a robust immune defense.