We are constantly exposed to infectious agents. Our immune system developed to counter infections, though it is also activated when reacting to non-infectious but potentially injurious agents. Unfortunately, our immune system can also be inappropriately activated, leading to autoimmune disease and allergy. Furthermore, our immune system may become compromised, causing a limited or even absent ability to mount an immune response against an infectious agent. This article, the fourth in a series on general pathology published in Optometry in Practice, will cover the basics of the immune system components and its mechanisms, infection, and what happens when the immune system goes wrong – immunopathology and immunodeficiency.