Q.Write a note on Vaccination.

BCG (Bacillus Calmette–Guérin) is the vaccine given against tuberculosis.

BCG is prepared from a live, attenuated (weakened) strain of Mycobacterium bovis, and is administered (usually at birth in India) to confer immunity against tuberculosis, caused by Mycobacterium tuberculosis.

Vaccines can be made using attenuated (weakened live) pathogens, dead pathogens, purified surface antigens, or inactivated toxins (toxoids); the oral polio vaccine used in mass immunisation programmes (like India's Pulse Polio) is prepared from an attenuated (live but weakened) strain of poliovirus.

Immunisation works by exposing the body to a modified/harmless form of a pathogen so that the immune system generates memory cells without causing the actual disease. Different vaccine types use different strategies:

• Attenuated (live, weakened) pathogens: the pathogen is weakened so it can no longer cause disease but still stimulates a strong, long-lasting immune response — e.g. BCG (for TB) and the oral (Sabin) polio vaccine.
• Dead/inactivated pathogens: the whole pathogen is killed but its antigenic structure is preserved — e.g. the injectable Salk polio vaccine.
• Toxoids: inactivated bacterial toxins — e.g. tetanus and diphtheria vaccines.

Since the widely administered oral polio vaccine (used in India's national Pulse Polio programme, the version most commonly referenced in Indian school textbooks) is the live, attenuated (Sabin) type, the best-fit answer here is attenuated virus.

(a) antigen — the inactivated organism itself serves as the antigen, which is combined with a diluent and an adjuvant to formulate the vaccine.

'A' in the flow diagram is the antigen (the inactivated organism itself).

In vaccine manufacturing, disease-causing organisms are first grown in culture and then inactivated (killed) or attenuated, so they can no longer cause disease but still retain their surface antigenic determinants. This inactivated organism preparation is, itself, the antigen — the substance that will provoke a protective immune response when administered. To formulate the final vaccine, this antigen is combined with a diluent (to bring it to the correct concentration/volume for administration) and an adjuvant (a substance that boosts and prolongs the immune response to the antigen). So, following the diagram's arrows — 'Inactivate organisms' leading into box 'A', which then combines with diluent and adjuvant to give the finished 'Vaccine' — 'A' must represent the antigen itself, not an antibody, antitoxin, or antibiotic (none of which are produced by inactivating the pathogen).