The vaccine allows an antibody to be introduced into the body. This strongly resembles the pathogen to be fought but it is not at all dangerous for the patient.

There are three main types of vaccine:

♦ Attenuated vaccines contain live infectious agents that have lost the majority of their pathogenic properties. For instance, this is the case of the vaccine against yellow fever.

♦ In many cases, the infectious agent is killed by a physicochemical procedure and then more or less purified. For instance, this is how the flu vaccine is produced. This is called an inactivated vaccine.

♦ Finally, the new generation of vaccine is purely synthetic. Coding DNA for the protein of the microbe is inserted into a yeast or a laboratory bacterial strain and the protein is produced in large quantities. This is called a recombinant vaccine. This is the case for the vaccine against hepatitis B and against infectious mononucleosis.

Once in the body, this inoffensive viral particle will trigger an immune reaction. B lymphocytes (a type of white blood cell) produce a mass of antibodies. These will combine with the surface of the foreign body at sites call antigens. The foreign body is thus neutralised. At the same time, the body generates memory cells. These act as sentinels and allow the body to react more quickly and more effectively if the virus or bacteria is present.

This is what vaccinations are all about: by "fooling" the body with a fake infectious agent, the vaccine prepares the body to better protect itself against a possible future infection by the real pathogen.