Immunity is defined as resistance to disease, specifically infectious disease. The collection of cells, tissues, and molecules that mediate resistance to infections is called the immune system, and the coordinated reaction of these cells and molecules to infectious microbes is the immune response.
The importance of the immune system for health is dramatically illustrated by the frequent observation that individuals with defective immune responses are susceptible to serious,
often life-threatening infections. Conversely, stimulating immune responses against infections by vaccination is the most effective method for protecting individuals against
infections. For example this approach helped to eradication of smallpox, this is the only deisease that has been eliminated from civilization by human. Immune responses also participate in the clearance of dead cells and in initiating tissue repair.
In contrast to these beneficial roles, defloration in immune responses cause to many inflammatory diseases with serious morbidity and mortality. Moreover, the immune response is the major barrier to successful organ transplantation. The products of immun system are of great medical use; antibodies (certain cells of immune system) are used in laboratory testing and in research as highly specific reagents for detecting a wide variety of molecules in the circulation and in cells and tissues. Antibodies designed to block or eliminate harmful molecules and cells are in wide-spread use for the treatment of immunologic diseases, cancers, and other types of disorders.
Short introduction to Innate and Adaptive Immunity
Host defense mechanisms consist of innate immunity, which mediates the initial protection against infections, and adaptive immunity, which develops more slowly and provides more specialized, effective and longer defense against infections.
Innate immunity, (also called natural immunity or native immunity) is always present in healthy individuals prepared to block the entry of microbes and rapidly eliminate microbes that do succeed in entering host tissues.
Adaptive immunity, also called (specific immunity or acquired immunity) requires
expansion and differentiation of lymphocytes in response to microbes before it can provide
effective defense. It adapts to the presence of microbial invaders. Innate immunity is older but adaptive immunity is more specialized, powerful and evolved later.
The first line of defense in innate immunity is provided by epithelial barriers and by cells and natural antibiotics present in epithelia, all of which function to block the entry of microbes. If microbes do breach epithelia and enter the tissues or circulation, they are attacked by phagocytes, specialized lymphocytes called natural killer cells, and several plasma proteins, including the proteins of the complement system. All these mechanisms of innate immunity specifically recognize and react against microbes.Also innate immune responses enhance adaptive immune responses against the
infectious agents.
Defense against infectious microbes additionally requires adaptive immune responses, espe-
cially with microbes that are pathogenic for humans and may have evolved to resist innate immunity.
The adaptive immune system consists of lymphocytes and their products, such as antibodies. Whereas the mechanisms of innate immunity recognize structures shared by classes of microbes, the cells of adaptive immunity (lymphocytes) express receptors that specifically recognize a much wider variety of molecules produced by microbes as well as noninfectious substances. These substances are called antigens. Adaptive immune responses often use the cells and molecules of the innate immune
system to eliminate microbes, and adaptive immunity functions to greatly enhance these antimicrobial mechanisms of innate immunity. For instance, antibodies bind to microbes, and these
coated microbes rapidly bind to and activate phagocytes (a component of innate immunity), which ingest and destroy the microbes.
Adaptive and Innate immunity will be discussed in later chapters
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