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.