Vaccination:

Now  let us see how vaccination protects us from disease. We have mentioned above that the W.B.Cs.  produce antibodies which neutralise the toxins produced by the invaders. These W.B.Cs.  are of different kinds and each kind consists of millions of cells which recognise and combat a specific foreign  invader. Once a class of W.B.Cs.  has encountered a particular  kind of invader, it develops memory and is thus trained to ward off future attacks. In this way the body becomes "immune"  to that infection and the process is called immunisation. Thus, our body regularly develops  natural immunity as a variety of fighting cells are produced by actual attacks of  infectious agents. Artificial immunisation is a clever  idea. It is done through "vaccination", that is by artificial introduction into  the body of a weak infection, which triggers off a defence mechanism, and produces W.B.C.  trained to combat that particular infection.