Passive immunization is carried out by administration of human and animal sera, which serve as the readymade source of pre-pared antibodies against a particular pathogen. These are given to an individual to confer immediate protection against particu-lar pathogen. The immunity, however, is of short duration.
Normal human immunoglobulins include (a) pooled human immunoglobulin,and(b)specifichyperimmune immunoglobulin.
Pooled human immunoglobulin is prepared from normal human serum containing a high concentration of a particu-lar antibody. It is usually used for short-term prophylaxis of hepatitis A, measles, etc.
Specific hyperimmune immunoglobulin is prepared from patients convalescing from the disease or from individuals actively immunized against a specific disease. These hyperim-mune sera are available against a variety of diseases. Examples are human rabies immunoglobulin (HRIG) against rabies, human tetanus immunoglobulin (HTIG) against tetanus, hepatitis B immunoglobulin (HBIG) against hepatitis B, and varicella zoster immunoglobulin (ZIG) against varicella zoster.
These hyperimmune sera are usually given intramuscularly. They are usually not given shortly before or after active immu-nization in order to avoid inhibition of immune responses.
Animal sera are prepared in animals. These sera rich in anti-bodies are produced by active immunization of animals, such as horses. Antitetanus serum and antivenom sera are examples of such sera used against tetanus and snake bites, respectively. Disadvantages of these sera are that in some recipients they may cause serum sickness and anaphylaxis; moreover, they are eliminated much rapidly from humans.
Combined active and passive immunization is often carried out to confer slowly developing immunity and immediate passive immunity, respectively, against certain diseases, such as diphtheria, tetanus, and rabies.