Introduction 
The mammalian immune system is incredibly effective at fighting and preventing infection. In an environment literally saturated with pathogens and potential pathogens, most individuals experience only a few short-lived incidents of infection. Often, what defines an organism as pathogenic or non-pathogenic is simply its ability to evade the immune system. Similarly severe immunocompromise such as HIV infection or that brought about by chemotherapy demonstrates how effective the immune system is: individuals lacking a functioning immune system suffer many more and many more serious infections than a healthy individual. 
  
Divisions in the Immune System 
The Immune system is typically broken down in two ways: 
1. Innate and Acquired Immunity; and
2. Humoral and Cell-Mediated Immunity (CMI)
Both of these divisions are slightly false as the immune system is really one highly interactive system. However they are often used and are useful in understanding the different ways in which the body responds to different infections. 
  
1. Innate and Acquired Immunity 
By definition, Innate Immunity is immunity to infection that is present from birth. It is not antigen-specific and does not improve with exposure to an infection. It also includes what one might term non-immunological means of resisting infection; such as the skin and stomach pH which is a vital part of protecting the body from infection. Acquired Immunity (or adaptive Immunity) depends on lymphocytes. Both B- and T-lymphocytes are antigen specific. Each lymphocyte responds to one antigen only hence the term antigen-specific. For each antigen that the acquired immune system responds to, there is a lymphocyte with that antigen specificity. The ability of these single specific cells to multiply is a key feature of how the immune system works. 
This division is slightly artificial, however, as both the innate and the acquired systems use many of the same cells and plasma factors and the acquired system is built on the innate one. 
  
2. Humoral and Cell Mediated Immunity 
These terms apply mostly to the acquired immune system. Humoral means chemical immunity: by use of soluble factors found in blood plasma. Cell-mediated means that the immunity is derived from the properties of cells. This terminology is misleading as both the humoral and the cell-mediated immune systems both use cells and both use soluble factors to fight invading organisms. The definition depends on the mechanism by which the pathogen is killed. The humoral system uses antibodies to attack pathogens, whereas the primary mechanism of attacking pathogens in the cell-mediated arm is via cytotoxic T cells. The humoral immune system is primarily responsible for killing bacterial pathogens, whilst the cell-mediated system provides immunity to viruses. 

Clonal Selection Theory 
Clonal Selection Theory is the basis of adaptive (acquired) immunity. It was first proposed by Macfarlane Burnet in 19571- and he subsequently won the Nobel prize for his work.
Figure 1 is a simplistic illustration of how clonal selection works. Each of the naïve lymphocytes at the top of the diagram has a different coloured receptor, indicating the fact that each receptor binds to a different antigen. 
The binding of antigen to that receptor causes division of that cell and differentiation into effector cells and memory cells. These cells all have the same antigen receptor and hence the same antigen specificity. 

The effector cells are relatively short lived whilst the memory cells persist for many years. In the advent of subsequent activation by the same antigen the cells again divide and produce effector cells. 
In figure 1, the lymphocytes are B-lymphocytes as they produce antibody but the same principal applies to T-lymphocytes. The binding of antigen triggers replication of lymphocytes.

Cytokines
The cells of the immune system work in a coordinated fashion. This is possible because they communicate via small proteins known as cytokines. (Greek: cyto-cell, kine-communicate). One cell will release these proteins and they then bind to specific receptors on the target cell.  Some of the cytokines are known as interleukins (inter-between, leukin – white cell). The interleukins were numbered: IL-1, IL-2, IL-6 etc. Confusingly some of these also have alternative names, such as IL-15 which is also know as T-cell growth factor. This second name is quite useful, as it indicates the function of IL-15, namely to promote the growth of T-lymphocytes.
 
CD Molecules
There are over 200 CD molecules. They are known by a number, for example CD-4, CD-8, CD-52 etc. The names are entirely uninformative in terms of their function and are historic, reflecting that each of these molecules was identified by mono-clonal antibody technology. There were named in the order they were discovered. All of them are proteins or glycoproteins that are found on the surface of cells. 

 

 

The Enemy 

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