Tag: plants and animal 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. 

Parathormone of parathyroid gland increases the blood calcium level by binding to receptors present on DCT of the nephron to stimulate conditional reabsorption of calcium. Thyroid hormone does not have any specific target cell and stimulate metabolic rate by all cells. Glucagon regulates blood glucose level while growth hormone stimulate overall growth and development. Calcitonin hormone stimulates a temporary reduction in number and activity of osteoclasts (bone cells) and thereby reducing the blood calcium level. Thus, the correct answer is option A.

• Parathyroid hormone regulates calcium levels in the blood, largely by increasing the levels when they are too low. 
• It does this through its actions on the kidneys, bones and intestine. 
• Bone parathyroid hormone stimulates the release of calcium from large calcium stores in the bones into the bloodstream. Hence, Parathyroid maintains the calcium level in the blood by action on Bone, Kidney and Intestine.
So, the correct answer is 'All of the above'.

Thymus is well developed in new born child and grows upto the age of puberty after that it starts shrinking and almost disappears in old age.