The immune system is principally organized around three-layered defenses of increasing specificity – the physical barriers, innate immune system, and the adaptive immune system. The physical barriers form the outermost protection layer and include the skin, eyelids, mucous membranes, normal flow of urine, tears, linings of the mouth, lungs, and stomach acid, among others. Urine is included as a physical barrier because it drains out microorganisms and other pathogens that gain entry into the urinary tract.
The principal function of the physical barriers rotates around preventing exogenous or foreign agents such as bacteria and viruses from gaining entry into an organism. For example, the walls of the passages in the nose are encrusted with mucus, which traps microorganisms and prevents them from gaining entry through the windpipe.
The second layer of defense is the innate immune system, with its most fundamental function being that of providing an immediate, but non-specific response once an exogenous agent breaches the physical barriers and enters the organism. According to Getz, the innate response is generally triggered when the exogenous agents or pathogens are identified by pattern recognition receptors, or when strained cells dispatch panic signals for help. It is at this juncture when the various facets of the innate response, including the white blood cells (neutrophils and monocytes), killer T cells, NK cells, and the complement cascade, among others, act to engulf and destroy the invading pathogens.
Jawed vertebrates have a third layer of defense in their immune system – adaptive immunity. This layer, which is activated by the innate response after the exogenous agents successfully evade the innate response, functions to engulf and destroy the pathogen by improving its recognition of the pathogen. Owing to the fact that the adaptive immune system is able to retain the improved response long after the foreign agent has been eliminated in the form of immunological memory, it has another important role in that it is able to mount speedy and more spirited attacks each time such an agent is encountered. Some of the cells that are most active in destroying the invading pathogens include lymphocytes, killer T cells, helper T cells, B lymphocytes, and antibodies. All in all, it can be said that the above-discussed layers form the body’s defense mechanism against infection.