In other words, the antigens stimulate specific immune reactions from antibody-mediated B-lymphatic cells and cell-mediated T-lymphatic cells in moderate level.  As a result, when antigens strike the body naturally, the number of antibody against them has already increased, and cytotoxic T cells remember the antigens, all for quicker immune reaction.

The mechanism of vaccination is from the body’s specific immune reaction after the natural infection by antigens.  In 1798, Edward Jenner applied cowpox virus for vaccination against human’s smallpox.  Later on, Louis Pasteur developed vaccines against rabies as well.  From that point forward, the theories behind vaccinations started to be founded, and active use of vaccination became a major consideration.

Infective disease is a big part of illness and death.  Especially, children show the highest rate of infective disease.  Even in the end of the 20th century, millions of young lives worldwide are dying from bacterial or viral infections annually. Preventative methods against this situation include improvement of primary hygiene, such as blocking the pathway of antigens and proper sterilization.  However, a great financial support is needed in order for a nation to improve the hygiene of a large community, and therefore, such actions cannot solve the problem in a short period of time.