The immune system is a collection of different cell and tissue types and hundreds of different proteins that collectively protect the body from infection by viruses, bacteria, protists, fungi, and even cancer. While every living organism possesses internal mechanisms that protect it from attack by pathogens, the immune system is limited to multicellular organisms. The human immune system has evolved to be incredibly complex. Innate immunity is a nonspecific immune response, including chemical and physical barriers and internal cellular and chemical defenses.
The first and second lines of defense are considered the main components of innate immunity. They rapidly respond to infection and are not specifically tailored to fight one particular pathogen. The first line of defense is made up of the physical and chemical barriers that prevent entry of pathogens into the body. The skin or epidermis forms a water-resistant, physical barrier, while the mucous membranes line internal body cavities that are open to the external environment, such as the lining of the lungs, the intestines, and the nasal passages. These physical defenses are bolstered by several chemical defenses that prevent the growth of many microbial types. These chemical defenses include mucous, sweat, and lysozyme, an enzyme capable of destroying cell walls.
While the first line of defense is physical and chemical in nature, the second line of defense involves nonspecific biological reactions that prevent infections from taking hold. When pathogenic microbes survive to make it past the first line of defense, then the second line of defense is initiated. Immune cells constantly monitor for infection and if one is detected they stimulate a response in the form of inflammation, activation of phagocytic cells, and the complement system, a set of proteins capable of activating innate immune responses.
The third line of defense, also known as the adaptive immune response, is only found in animals and is the response to specific pathogens that have already breached the barriers. Adaptive immunity is the immune response that is specific to a particular pathogen and arises after an immunizing event, such as vaccination or an infection. A microbe-specific response is initiated using antigen molecules from a pathogen to train the immune system's T and B cells how to recognize the pathogen in a process called activation. Where innate immunity is immediate and general, adaptive immunity is specific and has memory. For example, after exposure to measles, an individual's body produces memory T cells and B cells that retain a copy of the measles antigen. Upon a subsequent exposure to measles, the memory cells quickly multiply to allow for a faster response.