Answer:
thymus
Step-by-step explanation:
The thymus is part of the endocrine system, but it is much more closely associated with the immune system. It is a small, flat gland located in the upper chest, behind the sternum. The thymus plays a vital role in the development of the immune system. It produces T cells, which are white blood cells that play a key role in fighting infection. The thymus also helps to regulate the immune system by producing hormones that suppress the activity of T cells that are not needed.
The thymus begins to shrink and lose its function after puberty. This is why older people are more susceptible to infection. However, there are ways to boost the function of the thymus, such as eating a healthy diet and getting regular exercise.