Basic Immunology

The Greek word “Immune” means “to be protected”. The protection offered by the introduction of various antigens or antibodies is called acquired immunity. The process by which this acquired immunity is obtained is known as `Immunization’. Immunization is of two types viz., active and passive. When specific antigens evoke the needed immune response in the system it is called active immunization and when antibodies are supplied ready made in the form of immune globulins and sera it is known as passive immunization. Each and every vaccine is selected based on three important criteria viz., necessity, safety and efficacy. And a vaccine thus selected has already undergone the following three basic trials before it is licensed:

Phase I Trial (Human volunteers, for tolerance, safety)
Phase II Trial (Human volunteers, for immune response, safety)
Phase III Trial (For Field efficacy, safety)

Further, before a vaccine is actually marketed it undergoes the sterility, purity and potency tests at the level of the manufacturer and the National Control Authority.

The pathogenic infectious agent induces disease and the host immune system responds with immunity, first to ensure recovery and secondly to offer protection from disease if the same pathogen is encountered again. A vaccine is composed of one or more antigens of the pathogen, which will induce protective immune response within the risk of the disease itself.

Vaccines consist of attenuated live organisms (eg. Oral Polio vaccine, Oral Typhoid vaccine, Varicella Vaccine, Measles vaccine), whole organisms in a killed form (eg. Pertussis vaccine, Whole cell typhoid vaccine, Rabies vaccine, Killed polio vaccine), modified exotoxins called toxoids (eg. diphtheria toxoid, tetanus toxoid), or subunit vaccine (eg. polysaccharide capsular antigens of S.typhi or Haemophilus influenzae type b and the surface proteins of hepatitis B virus).

The Vaccine mimics the infection with the respective pathogen, but without risk of the disease. The consequent immune response may be manifested through antibody (humoral immunity) or cell mediated immunity (CMI), or both. If the antigen stimulates Th1 series of T helper lymphocytes, then stronger cytotoxic lymphocytic response is obtained; if Th2 series is stimulated, the ultimate expression of immunity is predominantly humoral. Carbohydrate antigens are T cell independent; hence they stimulate B cells directly without T helper cell modulation. The result is predominantly IgM response without IgG production or induction of immunological memory. BCG elicit CMI. Maternal CMI is not transferred to the foetus. Therefore BCG can be given at birth, OPV is given by mouth; it establishes local infection in a proportion of children. Maternal antibody in the infant’s circulation is a very weak inhibitory factor; hence OPV also can be given at birth. Hepatitis B surface antigen is an excellent immunogen, overcoming, to a large extent, the inhibiting effect of maternal antibody; hence that too can be given at birth. On the other hand, live Measles vaccine may be completely inhibited in the presence of detectable maternal antibody in the infants circulation. Therefore Measles vaccine is given after a delay of 9 months from birth and MMR only after 12 months.

To be successful in protective role, vaccines should be given before the age when the infection itself is anticipated. In the case of tetanus, neonatal tetanus cannot be prevented by immunization of the infant. Therefore high maternal antibody levels are ensured by immunizing the pregnant women. The transplacental antibodies protect the neonate.

14 The currently used childhood vaccines do not have any interference with each other. Therefore they can be given simultaneously. Several antigens can be given the same day. Between any two immunization sessions, 4 weeks interval is recommended. Between 2 doses of the same vaccine, a minimum interval of 4 weeks should be observed. Increasing the interval between doses to 2 or 3 months actually improves antibody levels. However, completion of immunization at the earliest age is achieved with a minimal interval of 4 weeks and the drop out rate also becomes less. BCG and OPV may be given from the day of birth until 2 weeks of age, so that there would be 4 weeks gap until the next contact for immunization at 6 weeks. If the opportunity to give BCG was not available in the neonatal period, it may be given at 6 weeks, simultaneous with DPT and OPV.

One should be familiar with the exact definition of certain terms in vaccinology, to understand basic immunology, better:

Each time a dose of a vaccine is given, the doctor should explain to the mother the nature of vaccine, the number of doses needed, the disease to be prevented, adverse reactions and their probability and treatment, and the date due for the next session of immunization.