The double-lobed mass of tissue located in the mediastinum along the trachea behind the sternum that is involved in immunity is known as the

The correct answer is thymus gland.

Explanation:

In the immune system, the thymus is considered as a primary lymphoid organ. Thymus is present behind the sternum, above the heart and between the two of our lungs. It is made up of two-lobed and is involved in releasing hormones and providing immunity.

Thymus is the primary site where T- cells become mature. T- cells are formed in bone marrow but reaches to thymus for maturation. These mature T- cells play a very important role in providing adaptive immunity against foreign molecules.  

Thymus is larger till puberty and it slowly shrinks as the age increases. So the correct answer is thymus.

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the oldest known insects are believed to live around 400 millions ago as suggested by the fossil records.   some plants having woody stems also evolved during this time.

after the evolution of insects the first four legged species or the tetrapods evolved in shallow freshwater habitats from the intermediate species approximately 397 million years ago. the tetrapods went on conquering the land giving rise to amphibians, reptiles, mammals and birds.

the major split in tetrapods first occurred 340 million years ago branching off from amphibians.

363 million years ago when the carboniferous period began the insects started roaming on land and taking flights, following which the four limbed tetrapods adapted in the terrestrial habitat.