At age 85, lyle's immune system does not respond to vaccines as well as it did when he was younger. the atrophy of which endocrine gland is likely responsible for this

The thymus.The thymus plays an important role in the development of T-lymphocytes which is an important type of white blood cell. It reaches maximum development at puberty and then the thymus begins to get atrophied (the shrinking of the thymus with age) and then the thymic stroma is replaced by fat tissue. Loss of thymic function is thought to contribute to weaker immune system of the elderly, increasing instances of diseases such as cancers and other infections.