The human immune system consists of lymphoid organs, tissues, cells and soluble molecules like antibodies. As you have read, immune system is unique in the sense that it recognises foreign antigens, responds to these and remembers them. The immune system also plays an important role in allergic reactions, auto-immune diseases and organ transplantation. Lymphoid organs: These are the organs where origin and/or maturation and proliferation of lymphocytes occur. There is lymphoid tissue also located within the lining of the major tracts (respiratory, digestive and urogenital tracts) called mucosaassociated lymphoid tissue (MALT). It constitutes about 50 per cent of the lymphoid tissue in human body.

Which of the following organ keeps on decreasing in size with age?

Thymus

Thymus is a site of differentiation of T-cells . The thymus is quite large at the time of birth but keeps reducing in size with age and by the time puberty is attained it reduces to a very small size.