Immune System

 Immune System Description

The immune system is the system of specialised cells and organs that protect an organism from outside biological influences. (Though in a broad sense, almost every organ has a protective function - for example, the tight seal of the skin or the acidic environment of the stomach.) When the immune system is functioning properly, it protects the body against bacteria and viral infections, destroying cancer cells and foreign substances. If the immune system weakens, its ability to defend the body also weakens, allowing pathogens, including viruses that cause common colds and flu, to grow and flourish in the body. The immune system also performs surveillance of tumor cells, and immune suppression has been reported to increase the risk of certain types of cancer.

The immune system is often divided into two sections:

Innate immunity: Comprised of hereditary components that provide an immediate "first-line" of defense to continuously ward off pathogens.
Adaptive (acquired) immunity: By manufacturing antibodies (a type of protein) and T-cells specifically designed to target particular pathogens, the body can develop a specific immunity to particular pathogens. This response takes days to develop, and so is not effective at preventing an initial invasion, but it will normally prevent any subsequent infection, and also aids in clearing up longer-lasting infections.  

Structure

Most multicellular organisms possess an "innate immune system", generally comprising a set of germ-line encoded receptors to pathogens, that does not change during the lifetime of the organism. Adaptive immunity, in which the responses to pathogens change and develop during the lifetime of an individual, seems to have appeared somewhat abruptly in evolutionary time, with the appearance of chondrichthyes (cartilaginous or jawed fish).

Organisms that possess an adaptive immunity also possess an innate immunity, and with many of the mechanisms between the systems being common, it is not always possible to draw a hard and fast boundary between the individual components involved in each, despite the clear difference in operation. Higher vertebrates and all mammals have both an innate and an adaptive immune system.

Innate Immune System

The adaptive immune system may take days or weeks, after an initial infection, to have an effect. However, most organisms are under constant assault from pathogens, which must be kept in check by the faster-acting innate immune system. Innate immunity defends against pathogens by rapid responses coordinated through "innate" receptors that recognize a wide spectrum of conserved pathogenic components. Plants and many lower animals do not possess an adaptive immune system, and rely instead on their innate immunity.

The study of the innate immune system has recently flourished. Earlier studies of innate immunity utilized model organisms that lack adaptive immunity, such as the plant Arabidopsis thaliana, the fly Drosophila melanogaster, and the worm Caenorhabditis elegans. Recent advances have been made in the field of innate immunology with the discovery of toll-like receptors (TLRs) and the intracellular nucleotide-binding site leucine-rich repeat proteins (NODs), which are receptors in mammal cells that are responsible for a large proportion of the innate immune recognition of pathogens.

In 1989, prior to the discovery of mammalian TLRs, Charles Janeway conceptualized and proposed that evolutionarily conserved features of infectious organisms were detected by the immune system through a set of specialized receptors, which he termed pathogen-associated molecular patterns (PAMPs) and pattern recognition receptors (PRRs), respectively. This was a remarkable insight at the time but was only fully appreciated after the discovery of TLRs by the Janeway lab in 1997. The TLRs now comprise the largest family of innate immune receptors (or PRRs). Janeway’s hypothesis has come to be known as the ‘stranger model’ and substantial debate in the field persists to this day as to whether or not the concept of PAMPs and PRRs, as described by Janeway, is truly suitable to describe the mechanisms of innate immunity. The competing ‘danger model’ was proposed in 1994 by Polly Matzinger and argues against the focus of the stranger model on microbial derived signals, suggesting instead that endogenous danger/alarm signals from distressed tissues serve as the principle purveyors of innate immune responses.