VACCINES
T-cell memory is very important for long-lasting immunity, because T-cells control both humoral and cell mediated immunity. When the immune system recognizes a foreign antigen for the first time, an immune response is produced. When T cells are involved, immunological T-cell memory is produced. When the body encounters same antigen subsequently, a stronger immune response is produced. This is because of existing immunological memory against that antigen. Further antigenic stimulus increases the immune response. First antigenic stimulus is “priming” whereas subsequent stimuli are “booster”. This is the principle of active immunization.
The term “vaccine” was coined by Louis Pasteur to commemorate first successful immunization against small pox by Edward Jenner. The term vaccine was derived from “vacca”, meaning cow, since Edward Jenner used cowpox virus (Vaccinia) to prevent smallpox infection. Vaccination involves deliberate exposure to antigen under conditions where disease should not result. Vaccination is aimed at inducing active immunity in an individual, so that subsequent contact with the microorganism following natural infection induces strong protective immune response. The protective immunity may involve secretion of neutralizing antibodies or production of memory CTL or Th1 cells. The use of vaccines is now being extended to immunize against tumors or to block fertilization (contraceptive vaccines). A vaccine is a suspension of whole (live or inactivated) or fractionated bacteria or viruses that have been rendered nonpathogenic, and is given to induce an immune response and prevent disease. Even though no vaccine is entirely safe or completely effective, their use is strongly supported by their benefit-to-risk ratio.
Properties of ideal vaccine:
• Provide long lasting immunity.
• Should induce both humoral and cellular immunity.
• Should not induce autoimmunity or hypersensitivity.
• Should be inexpensive to produce, easy to store and administer.
• Vaccines must also be perceived to be safe.
The vaccine vial may contain relevant antigen, adjuvant (usually alum), preservatives and/or traces of protein derived from the cells in which the vaccine agent was cultured e.g. egg protein
