A. How does Immune System respond to Antigen? Human Immune system is responsible for protecting our body from pathogens such as viruses, bacteria, and microbes. The immune system is composed of a collection of cells, which operate autonomously and through interaction with each other to create complex series of events leading to the destruction of pathogens. At a high level, cells can be categorized into two [7] groups: detectors and effectors. Detectors identify pathogens, and effectors neutralize them. The Immune system induces two kinds of immune responses: innate response and adaptive response. During innate immune response process a specialized class of cells called phagocytes (macrophages and neutrophils) is used to detect pathogens instantly in the body. However, it is incapable of recognizing unknown viruses and bacteria that mutate and evolve.