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INTRODUCTION

1.  Antigen- Antibody Interactions

The utilization of the in vitro reaction between antigen and serum antibodies (serology) serves as the basis for many immune assays. Because of the exquisite specificity of the immune response, the interaction between antigen and antibody in vitro is widely used for diagnostic purposes, for the detection and identification of either antigen or antibody. An example of the use of serology for the identification and classification of antigens is the serotyping of various micro organisms by the use of specific antisera.

The interaction of antigen with antibodies may result in a variety of consequences, including precipitation (if the antigen is soluble), agglutination (if the antigen is particulate), and activation of complement. All of these outcomes are caused by the interactions between multivalent antigens and antibodies that have at least two combining sites per molecule. The consequences of antigen - antibody interaction listed above do not represent the primary interaction between antibodies and a given epitope but, rather, depend on secondary phenomena, which result from the interactions between multivalent antigens and antibodies. Such phenomena as the formation of precipitate, agglutination, and complement activation would not occur if the antibody with two or more combining sites reacted with a hapten (i.e., a unideterminant, univalent antigen), nor would they occur as a result of the interaction between a univalent fragment of antibody, such as Fab, and an antigen, even if the antigen is multivalent. Cross linking of various antigen molecules by antibody is required for precipitation, agglutination, or complement activation, and it is possible only if the antigen is multivalent and the antibody is divalent [either intact, or F(ab')2]. By contrast, no, cross linking is possible if the antigen or the antibody is univalent.

2.  Active Immunization or Vaccination

The terms vaccination and vaccine derive from the work of Edward Jenner who, over 200 years ago, showed that inoculating people with material from skin lesions caused by cowpox (L.vaccinus, of cows) protected them from the highly contagious and frequently fatal disease smallpox.

Since Jenner's time the term has been retained for any preparation of dead or weakened pathogens, or their products, that when introduced into the body, stimulates the production of protective antibodies or T cells without causing the disease. In molecular terms, the goal is to introduce harmless antigen (s) with epitopes that are also found on the pathogen.

Vaccination is also called active immunization because the immune system is stimulated to develop its own immunity against the pathogen. Passive immunity, in contrast, results from the injection of antibodies formed by another animal (e.g., horse, human ) which provide immediate, but temporary, protection for the recipient.

3.  Kinds of Vaccines

3. 1     Killed whole organisms

In the relatively crude approach, the vaccine is made from the entire organism, killed to make it harmless. The typhoid vaccine is an example.

3. 2     Attenuated organisms

Here, the organism has been cultured so as to reduce its pathogenicity, but still retain some of the antigens of the virulent form. The Bacillus Calmette - Guerin (BCG) is a weakened version of the bacterium that causes tuberculosis in cows. BCG is used as a vaccine against tuberculosis in many European countries but is rarely used in the U.S.

3. 3    Toxoids

In some diseases, diphtheria and tetanus are notorious examples, it is not the growth of the bacterium that is dangerous, but the protein toxin that liberated by it. Treating the toxin with, for example, formaldehyde, denatures the protein so that it is no longer dangerous, but retains some epitopes on the molecule that will elicit protective antibodies.

3. 4     Surface molecules

Antibodies are most likely to be protective if they bind to the surface of the invading pathogen triggering its destruction. Several vaccines employ purified surface molecules:

§    Influenza vaccines contains purified hemagglutinins from the viruses currently in circulation around the world.

§    The gene encoding a protein expressed on the surface of the  hepatitis B virus, called hepatitis B surface antigen or HBsAg, can now be expressed in E. coli cells and provides the material for an effective vaccine.

§    Hepatitis B infection is strongly associated with the development of liver cancer. Here there is the first vaccine against a cancer.

§    Some 80 different strains of Streptococcus pneumoniae cause pneumonia in humans. They differ in the chemistry of the polysaccharide capsule that surrounds them (and makes it difficult for phagocytes to engulf them by endocytosis) The current vaccine consists of tiny amounts of the purified capsular polysaccharides of the 23 most common and / or dangerous strains.

3. 5    Inactivated virus

Like killed bacterial vaccines, these vaccines contain whole virus particles that have been treated (again, often with formaldehyde) so that they cannot infect the host’s cells but still retain some unaltered epitopes. The Salk vaccine for polio (IPV) is an example.

3. 6     Attenuated virus

In these vaccines, the virus can still infect but has been so weakened that it is no longer dangerous. The measles, mumps, and rubella (“German measles”) vaccines are examples. The Sabin oral polio vaccine (OPV) is another example. It has advantages over the Salk vaccine in that

§    It can be given by mouth rather then by injection

§    It can spread to the other members of the vaccinee’s       family thus immunizing them as well.

It has the disadvantage of - on rare occasions-regaining full virulence and causing the disease. For this reason, the Salk vaccine has once again become the preferred vaccine in the U. S. A.

Following table describes some of the most widely - used vaccines that are used in human beings.