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The Immune System is the name of a collection of Molecules, Cells and Organs whose complex interactions form an efficient system that is usually able to protect an individual from both outside invaders and its own altered internal cells.
This combination of components is the product of millions of years of evolution, which has selected those Organisms that are best able to prevent their destruction by MicroOrganisms or Tumors.
This text attempts to clarify exactly what the Immune System does, and more importantly, the mechanism of its action.
It is difficult to clearly delineate the elements of the Immune System from the rest of the body; however, a traditional, if somewhat inadequate method, divides it into two functionally distinct parts: those elements which are Innate (Non-Adaptive), and those which are Acquired (Adaptive).
It is becoming apparent, these two groups are in no way isolated, a complex dialog is constantly going on between them. As such, some attention will also be given to the "Innate/Acquired Dialogue."
The T-Cell is one of the most important elements in the human Immune System. Its role in directly destroying infected or Cancerous cells is great, and it is the control center for the rest of the Acquired and Innate Immune System.
The numerous Cytokines it produces and its array of surface molecules are critical for the control of all other Immune elements.
As AIDS demonstrates, the destruction of the Helper T-Cell lineage results in the total collapse of the Immune System. But, it is important not to suggest any kind of T-Cell independence.
Many other cell types including Macrophages, NK Cells, and B-Cells are important in the Effector phase of the Immune Response. Macrophages and Dendrite Cells are also important regulators of Immune Responses.
The elements on these (and other) cells which are central to the control of T-Cells are the Major Histocompatibility Complex (MHC) molecules. Without these, there is no presentation of internal or external Antigens to the T-Cell.
Their concentration on a cell surface is a critical determinant of the degree of T-Cell activation. And, as their name suggests, they contribute to the T-Cell's ability to discriminate between self-cells and other.
The Helper T-Cells (CD4+) are probably the most important Immune Cells, despite the fact that by definition they do not directly destroy any diseased cells or MicroOrganisms.
Rather, they produce a plethora of chemical factors (Cytokines), and express many surface elements, thereby regulating all other aspects of the Immune System. They too act by recognizing MHC complexes with their TCRs.
These cells, whether they are B-Cells or Macrophages, or some other Leukocyte, consume foreign proteins (specifically or randomly), and present small portions (Peptides), in conjunction with MHC Class II.
If the complex they present is recognized by a CD4+ Cell, that cell will produce chemicals which effect the cell, which presented the MHC complex, as well as other Immune Cells.
The CD4+ Cell will go on to divide and mature, producing more Cytokines that activate other Effector Cells and Memory Cells, which function as with CD8+ and B-Cells.
No Acquired Immune Response could take place without the chemicals that a CD4+ Cell produces, and its absence, as seen in cases of HIV infection, results in a collapse of the Immune System.
As the CD4+ Cell illustrates, no matter how important any single part of the Immune System may be, none is effective, if it is unable to communicate with the whole.
It is the synergy of the combined Immune System, not the potency of any one element, which makes it so capable a defender.
This fact, has lead to the study of the various ways the Immune System interacts with itself and its environment. Although an enormous amount of work remains in this area, we have already learned a great deal.
This fact has been crucial to understanding, how the Immune System can respond to *damage* and *foreign*. Heat Shock Proteins activate both Innate and Specific elements, increasing Immune activity around the infection. (See: alphaB-Crystallin)
The Cytokine environment is very important in determining what kind of response is made, and its effectiveness.
Many Parasitic Diseases owe their success to their ability to induce the production of Cytokines, which lead to a response that is not harmful to the Parasite.
It also seems that the initial cells to respond to an infection, determine the future course of the response.
Natural Killer Cells may respond to an infection by producing one Cytokine, while Macrophages would produce another.
The knowledge that the initial site of infection is critical, has lead to the development of more effective vaccine adjuvants and inoculation methods.
This receptor enables Macrophages and CD8+ Cells to use Ig's secreted by B-Cells, to recognize specific foreign factors.
The "network" theory of Immune regulation, suggests an even larger role for Ig - that AntiBody to AntiBody communication is responsible for the activation and eventual suppression of some Immune responses.
In summary, while it is convenient for descriptive purposes to break the Immune System into Innate and Acquired branches, which themselves can be further sub-divided, this is a somewhat artificial division.
Both branches influence each other and are in turn shaped by their environment.