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.