The resistance that an individual acquired during life is known as acquired immunity as distinct from inborn innate immunity.
Acquired immunity is of two types, active and passive. Active immunity is the resistance developed by an individual as a result of an antigenic stimulus. It is also known as adaptive immunity as is represent an adaptive response of the host to a specific pathogen or other antigen.
Active immunity sets in only after a latent period which is required for the immunological machinery to be set in motion.
During the development of active immunity, there is often a negative phase during which the level of measurable immunity may actually be lower than it was before the antigenic stimulus. This is because the antigen combines with any pre-existing antibody and lowers its level in circulation. Once developed, the active is long-lasting.
If an individual who has been actively immunized against an antigen experiences the same antigen subsequently, the immune response occurs more quickly and abundantly than during the first encounter. This is known as secondary response.
Beside the development of humoral and cellular immunity, active immunity is associated with immunological memory.
Active immunity may be natural or artificial.
The resistance that is transmitted passively to a recipient in a ‘readymade’ form is known as passive immunity. In passive immunity there is no active role of recipient’s immune system. There is no antigenic stimulus; instead, preformed antibodies are administered. There is no latent period in passive immunity, protection being effective immediately after passive immunization. There is no negative phase.
Passive immunization is less effective and provides an immunity inferior to that provided by active immunization.
The main advantage of passive immunization is that it acts immediately and therefore can be employed when instant immunity is desired.