The organ primarily associated with the immune system is the thymus. The thymus plays a crucial role in the development of T-cells, a type of white blood cell essential for immune responses. Located in the upper chest, the thymus is most active during childhood and shrinks in size and activity with age. It is responsible for training T-cells to distinguish between the body’s own cells and foreign invaders, laying the foundation for a functioning immune system. A well-functioning thymus is vital for overall health and immunity, particularly during the early years of life when the body encounters various pathogens.