Someone has an immune disorder that causes dry mouth and lost sense of taste. How is this possible?

Sjorgen's. Sjorgen's syndrome is an autoimmune disease that affects the mucous membranes. The structures such as salivary and lacrimal glands get damanged its symptoms are dry eyes, loss of saliva, and loss of taste. It can also affect the kidneys, joints, and other organs.
Indirectly connected. Immune disorders generally do not cause loss of sense of taste directly. The medications being taken cause dry mouth. Saliva is required for taste buds to be connected to food particles to be able to process the feeling of taste. Depending upon the amount of dryness, the loss of taste sensation might vary. Drinking more water, using salivary flow enhancers, or artificial saliva might, help.