Thymus - class-XII

Removal of the thymus gland in the adult has little effect, but when the thymus is removed in the newborn, T cells in the blood and lymphoid tissue are depleted and failure of the immune system causes a gradual, fatal wasting disease. 

T cells or T lymphocytes are a type of lymphocyte, that plays a central role in cell-mediated immunity. They are called as T-cells because they mature in the thymus gland.

The thymus gland is an organ in the upper chest cavity, that processes lymphocytes, a type of white blood cell, that fights infections in the body. 

The thymus is a specialized organ of the immune system. Within the thymus, T cells or T lymphocytes mature. At birth, it is about 5 cm in length, 4 cm in breadth, and about 6 mm in thickness. The organ enlarges during childhood and atrophies at puberty. The thymus is at its largest in children. The thymus reaches maximum weight (20 to 37 grams) by the time of puberty. As one ages, the thymus slowly shrinks, eventually degenerating into tiny islands of fatty tissue. By the age of 75 years, the thymus weighs only 6 grams.

Hassall's corpuscles (or thymic corpuscles) are structures found in the medulla of the human thymus, formed from eosinophilic type VI epithelial reticular cells arranged concentrically. 

The triploblastic animals have three prominent germ layers known as ectoderm, endoderm and mesoderm. Thymus gland develops from two hollow elongations (third and fourth pharyngeal pouches) of endoderm which descends to the embryonic thorax by the end of eight weeks of gestation. The lymphoid cells of the gland originate from mesoderm. Thus, the correct answer is option B.

Endoderm, the innermost of the three germ layers, or masses of cells (lying within ectoderm and mesoderm), which appears early in the development of an animal embryo. The endoderm subsequently gives rise to the epithelium (tissue that covers, or lines, a structure) of the pharynx, including the eustachian tube, the tonsils, the thyroid gland, parathyroid glands, and thymus gland; the larynx, trachea, and lungs; the gastrointestinal tract (except mouth and anus), the urinary bladder, the vagina (in females), and the urethra. 

Thymectomy is the process in which the thymus gland is removed. Thymus gland is located in the chest area. Thymus gland is responsible for T-cell differentiation. The gland is recommended to be removed in cases of people suffering from Myasthenia gravis. The gland is removed in childhood, which generally have no adverse effects. However, when the gland is removed in adulthood, certain immune functions get affected. T-cells differentiation may be affected leading to improper production of T-cells.

Thymus is small lymphoid structure present in lower art of neck in front of trachea, below the thyroid gland. It is the primary source of lymphocytes before birth and is the site of maturation or development of T-lymphocytes which circulates in lymphoid tissues. 

Thymus becomes inactive after puberty and shrinks. Most of the glands is replaced by adipose tissue. It produces thymosin which stimulates the development of disease-fighting T cells. The immune system produces most of its T cells during childhood and requires very few new T cells after puberty so the gland degenerates.

T-lymphocytes are a type of white blood cell that matures in the thymus gland, which is why they are designated with the letter T.

Thymus glands is a specialised lymphoid organ of the immune system. It helps in the maturation of the T cells. it plays an important role during the puberty at the adolescence stage.  After puberty, it degenerates and it is replaced by fat. This shrinkage of thymus mainly takes place due to a high level of sex hormones.

Thymus gland is a prominent gland at birth but gradually atrophies in the adult. It is a soft, pinkish, bilobed mass of lymphoid tissue. It secretes a hormone thymosin which is involved in producing immunity.

Thymus is called temporary gland as it degenerates in old individuals resulting in a decreased production of thymosin.

The thymus gland, located behind our sternum and between our lungs, is only active until puberty.

Central tolerance is the tolerance established by deleting autoreactive lymphocyte clones before they develop into fully grown immunocompetent cells. It occurs primarily in the thymus.

B cells develop in the bone marrow but achieve maturity in
peripheral lymphoid organs. Developmental stages are defined by
the successful expression of heavy-chain and light-chain immunoglobulin genes. B cells provide humoral immunity against extracellular
pathogens through antibody production. Antibodies neutralize
pathogens and toxins, facilitate opsonization, and activate complement. The B-cell receptor or BCR is composed of immunoglobulin molecules that form a type 1 transmembrane receptor protein usually located on the outer surface of a lymphocyte type known as B cells.