|Innate immunity is the inborn immunity of the person. Innate immunity is non-specific in nature. The response of innate immune depends on the recognition of evolutionarily conserved structures present on pathogens, called pathogen-associated molecular patterns (PAMPs). The innate immune system is based mainly on physical and chemical barriers. There are three components of the innate immune system that includes anatomical barrier, humoral barrier and cellular barrier. Anatomical barriers called first line of defense includes skin, saliva, tears, mucus and cilia present in intestinal and respiratory tract. Skin act as a mechanical barrier whereas others are chemical barriers. If the pathogen escapes anatomical barriers acute inflammation occurs and humoral barrier comes into play. Humoral barriers include complement system and interleukin. Last is the cellular barrier that destroys the pathogen if it has entered the body by crossing the first two barriers. It includes various cells like neutrophils, macrophages, natural killer cells and dendritic cells.
When the pathogen encounters the cellular barrier then the process is such that firstly the neutrophils will encounter the pathogen and send signal throughout the body. The signals include complement products and cytokines. These chemical signals now invite other cells like macrophages and other immune cells and causes inflammation. Dendritic cells also become active and engulf the pathogen. If the pathogen is a virus the natural killer cells comes into play. These cells nonspecifically kill the cells that are infected by viruses or tumor cells. The healthy cells express MHC class 1 molecules on their surface so the natural killer cells destroy the cells that do not express MHC class 1 molecules.