The organ primarily associated with the immune system is the thymus. The thymus is a small organ located in the upper chest, just above the heart, and is crucial for the development of T-lymphocytes, a type of white blood cell vital for adaptive immunity. During childhood, the thymus produces and matures T-cells that respond to pathogens. As individuals age, the thymus shrinks and its activity decreases, which can affect the immune response. However, it plays a significant role in establishing a robust immune defense during the formative years of life.