Immunity Immunity: Types, Classification. Introduction to Immunity Immunity is the body’s defense mechanism that protects against pathogens (bacteria, viruses, fungi, and parasites) and foreign substances. It involves physical barriers, immune cells, and molecular responses to detect and eliminate harmful invaders. The immune system can be innate (natural) or adaptive (acquired). It includes white blood cells (WBCs), antibodies, cytokines, and complement proteins that work together to maintain immunity. Classification of Immunity