The thymus is an organ located in the upper chest that plays a key role in the development of T cells. It grows larger during childhood and reaches its maximum size at puberty. After puberty, the thymus slowly shrinks and is replaced by fatty tissue as a person ages. Within the thymus, immune cells called thymocytes mature into T cells that then travel to other parts of the body to help direct the adaptive immune system.

1. Thymus
 The thymus is a specialized primary lymphoid organ of the immune system. T
cells mature occurs inside the thymus.
 In children the thymus is grayish-pink in colour and in adults it is yellow.
 At birth it is about 4–6 cm long, 2.5–5 cm wide, and about 1 cm thick. The thymus
reaches maximum weight (20 to 37 grams) by the time of puberty.
 The thymus enlarges during childhood, and atrophies at puberty. With aging the thymus
slowly shrinks, eventually degenerating into tiny islands of fatty tissue.
 The thymus is composed of two identical lobes and is located anatomically in the anterior
superior mediastinum, in front of the heart and behind the sternum.
 Each lobe of the thymus can be divided into a central medullaand a
peripheral cortex which is surrounded by an outer capsule.
 The cortex and medulla play different roles in the development of T cells.
 In the two lobes, hematopoietic precursors from the bone-marrow, referred to
as thymocytes, mature into T cells.
 Once mature, T cells emigrate from the thymus and make up the peripheral T cells
responsible for directing many parts of the adaptive immune system.
 Loss of the thymus at an early age through genetic mutation results in
severe immunodeficiency and subsequent high susceptibility to infection.