The vaccine for tuberculosis contains a dead form of the tuberculosis bacterium. how does the body respond to the vaccine? How is passive immunity brought about? When is it needed?

• Let us consider the first part of your question. Dead bacteria of a particular disease are introduced in blood, which will be seen as a foreign invasion by the body. Macrophages of blood will soon engulf the dead bacterial cells.

After digestion of bacterial cells some of its surface antigens are displayed by macrophages on their membrane so that circulating lymphocytes become exposed to the foreign antigens.

Once exposed, lymphocytes will remember the foreign antigen (in this case surface antigen of tuberculosis bacterium) and would be able to produce specific antibodies against the germs in all events of future invasions.

This way a vaccine can immunise us, against a particular disease causing microbe, for life. This can happen naturally: as we suffer from diseases like measles or chicken pox, we become imunised for life against those particular diseases. Our body always remains prepared to fight off those germs to which our lymphocytes were once exposed: either naturally or through vaccination. This is called active immunity.

• Now let me discuss the second part of your question, which is about passive immunity. It is a short term immunity acquired by the body: either naturally, i.e. after birth antibodies are delivered to baby through mother's milk or artificially, by injecting antibodies contained in serum.

Injection of serum is essential to prevent death in case of snake biting or to prevent rabies after dog/cat biting.