Q.Define immunity and immune system.

The thymus enlarges in childhood, regresses with age, and is central to developing the body's T-cell-based immunity.

The thymus is a soft, bilobed gland located near the heart and behind the sternum. It is comparatively large at birth, continues to increase in size up to puberty, and thereafter progressively decreases in size and is largely replaced by fatty/connective tissue in adulthood.

Role in infection/immunity:

• The thymus secretes hormones called thymosins, which are responsible for the differentiation of lymphocytes (T-cells) into immunologically competent T-lymphocytes.

• T-lymphocytes matured in the thymus provide cell-mediated immunity — they directly attack infected/foreign cells and also help regulate (activate/suppress) other immune cells.

• Because the thymus is most active and largest during childhood, it plays a critical role in building up the immune repertoire early in life, helping the growing child fight infections; the gland is essentially non-functional in older individuals, which is one reason immune responses to new infections can weaken with age.

Immunity = the body's capacity to resist pathogens; the immune system = the network of cells, tissues and organs that provides that defence.

These two closely-related terms from the Immune System / Human Health and Disease topic of NCERT Class 12 Biology are defined as:

• Immunity: the ability of the host (body) to fight against and protect itself from disease-causing organisms (pathogens) and their toxins, as well as other foreign (non-self) substances. It may be innate (present from birth) or acquired (developed after exposure).

• Immune system: the body's defence system — a coordinated arrangement of specialised lymphoid organs (thymus, spleen, lymph nodes, bone marrow), cells (lymphocytes, macrophages and other white blood cells) and molecules (antibodies) — that recognises foreign material and mounts a response to eliminate it.