what is the principle of immunisation?

When the immune system first encounters an infectious agent, it responds against it and remembers it specifically. Hence the next time that microbe or its close relative is encountered, the immune system responds with renewed vigour and eliminates the infection even more quickly than the first time. This is the basis of immunisation principle. In immunisation, the immune system is fooled into developing a memory for a particular infection by introducing something that mimics the microbe into the body. This does not actually cause the disease, but protects the body from future exposure to the same microbe.