The immune system serves to defend against pathogens: microorganisms that attempt to invade and cause disease in a host.
The innate immune response has physical and chemical barriers that exist as the first line of defense against infectious pathogens.
Upon pathogen entry to the body, the innate immune system uses several mechanisms to destroy the pathogen and any cells it has infected.
Natural killer cells are part of the innate immune response that recognize abnormal MHC I molecules on infected/tumor cells and kill them.
Around 20 soluble proteins comprise the complement system, which helps destroy extracellular microorganisms that have invaded the body.
B and T cells, parts of the adaptive immune response, contain receptors that can identify antigens derived from pathogens.
The humoral immune response defends against pathogens that are free in the blood by using antibodies against pathogen-specific antigens.
Cell-mediated immunity involves cytotoxic T cells recognizing infected cells and bringing about their destruction.
The lymphatic system houses large populations of immune cells which are released upon detection of a pathogen.
Immunological memory allows the adaptive immune system to very rapidly clear infections that it has encountered before.
Immune tolerance of self and harmless antigens occurs by deleting B and T cells that recognize those antigens, often near mucosal surfaces.