How Vaccines Work
First let us see what happens when we contract an infection. The obvious reactions associated with the illness are pain, fever, rashes, vomiting, etc. What is not so obvious is that the body mounts its own defence against the disease. If this is adequate, the patient recovers. If not, the patient succumbs.
What is this defence mechanism? The disease causing organisms contain proteins called "antigens" which stimulate the immune response. The resulting immune response is multi-fold and includes the synthesis of proteins called "antibodies." These proteins bind to the disease-causing organisms and lead to their eventual destruction. In addition, 'memory cells' are produced in an immune response. These are cells that remain in the blood stream, sometimes for the life span of the host, ready to mount a quick protective immune response against subsequent infections with the particular disease-causing agent, which induced their production. If such an infection were to occur, the memory cells would respond so quickly that the resulting immune response could inactivate the disease-causing agents, and the symptoms would be prevented. This response is often so rapid that infection does not develop and the person is immune to the infection.
Vaccines stimulate the production of protective antibodies and other immune mechanisms. The immunity the body acquires naturally, or by vaccines, producing antibodies, which have a specific action on the germs of a specific disease, is called Active Immunity. This depends on the humoral and cellular response of the infected person.
When antibodies produced in one body are introduced into another (by vaccination, through placenta or breast milk, or transfer of lymphocytes) to fight disease, it is called Passive Immunity. Here the infected person gets the defensive ammunition readymade. Immunity is quick but short-lived, only till the current bout of infection is present. The patient's own immune system has no memory of the infection and cannot tackle it as quickly the next time round.