The main organ involved in the human immune system is the thymus. The thymus is crucial for the development and maturation of T-lymphocytes (T-cells), a type of white blood cell that plays a key role in mediating immune responses. Located in the chest, just behind the sternum, the thymus is most active during childhood and begins to shrink after puberty. The proper functioning of the thymus is essential for a well-functioning immune system, as T-cells are vital for identifying and responding to pathogens.