Cell-Mediated Immunity: Active and Passive | Immunology

In this article we will discuss about the active and passive immunity of cell-mediated.

1. Active Immunity:

On the basis of immunity acquired, active immunity may be natural or artificial.

i. Natural active immunity:

It results from either a clinical or an in-apparent subclini­cal infection by a microbe after repeated exposure to small doses of the infecting organism, which passes unnoticed.

Such immunity is usually long-lasting and plays important roles in preventing epidemics but duration of immunity varies with the pathogen.

For example, poliomyelitis, chicken-pox, influenza etc.

ii. Artificial active immunity:

It is the resis­tance produced by vaccination. The vac­cines are preparations of live, attenuated or killed microorganisms, or their anti­gens or active materials derived from them (e.g. toxoids).

2. Passive Immunity:

The resistance that is induced in the recipient by transfer of preformed (readymade form) anti­bodies against infective agent or toxin in another host, is called passive immunity.

Characteristics of Passive Immunity:

Immune system of a recipient does not play any active role in passive immunity. There is no antigenic stimulus. Instead, preformed antibodies are administered. The protective mechanism comes into force immediately after transfer of antibodies. The immunity is transient, usually lasting for days or weeks, only till the passively transferred antibodies are metabolised and elimi­nated.

No secondary type response occurs in passive immunity. Because, when a foreign antibody is administered a second time, it is eliminated more rapidly than initially.

Following first injec­tion of an antibody, such as immune horse serum, the elimination is only by metabolic breakdown, but during subsequent injections of horse serum, elimination is much quicker as it combines with antibodies to horse serum that would have been produced following its initial injections. This fac­tor of immune elimination limits the usefulness of repeated passive immunisation.

Advantage of Passive Immunity:

Since, passive immunity acts immediately, it can be employed when instant immunity is desired.

Types of Passive Immunity:

(i) Natural passive immunity:

It is the resis­tance passively transferred from mother to foetus and infants. For example, transfer of water anti­body (IgA) to foetus trans-placentally and to infant through milk (colostrum).

(ii) Artificial passive immunity:

It is the resistance passively transferred to a recipient by the administration of antibodies. The agents used for this purpose, are hyper-immune sera of animal or human origin and pooled human gamma glo­bulin or convalescent sera.

For example, anti-tetanus serum (ATS) pre­pared from hyperimmunised horses and pooled human gamma globulin or convalescent sera are used for passive immunisation against viral hepatitis A.