Found this online definition of immune system interesting:
"To function properly, an immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue."
The immune system of a metaphysician likewise must detect agents foreign to his own healthy state, his true being. This requires that he first be clear on what is that state is, and then be faithful in identifying with it - standing porter at the door of thought to reject anything unlike it.
Healthy state? Spirit, spiritual. God is Spirit, therefore His image and likeness is Spirit. I am ageless, diseaseless, deathless because God made me so! This kind of thinking serves as a super immune system. It is active, wise, and impenetrable.