CBSE Class 9 Answered

A vaccine is an antigenic preparation that is injected or swallowed and causes the development of active immunity in the patient.

Serum is the clear yellowish fluid obtained upon separating whole blood into its solid and liquid components after it has been allowed to clot. Also called blood serum. Blood serum from the tissues of immunised animals, containing antibodies is used to transfer immunity to another individual.

The bacteria which is used for preparing the vaccine is the causative agent itself. They are treated before being introduced to the body of the person, in order to make them relatively harmless. The vaccines are made as killed virulent organisms or as live non-virulent strains. For example - BCG vaccine is used against tuberculosis.