Section 1: General Aspects of Vaccination How Do Vaccines Mediate Protection?

Disease control or elimination requires the induction of protective immunity in a suffi cient proportion of the population. This is best achieved by immunization programs capable of inducing long-term protection, a hallmark of adaptative immunity that contrasts to the brisk but short-lasting innate immune responses. Long-term immunity is conferred by the maintenance of antigen-specifi c immune effectors and/or by the induction of immune memory cells that may be suffi ciently effi cient and rapidly reactivated into immune effectors in case of pathogen exposure. Vaccine-induced immune effectors (Table 2–1) are essentially antibodies—produced by B lymphocytes—and capable of binding specifi cally to a toxin or a pathogen. Other potential effectors are cytotoxic CD8 T lymphocytes* (CTL) that may limit the spread of infectious agents by recognizing and killing infected cells or secreting specifi c antiviral cytokines. The generation and maintenance of both B and CD8 T cell responses is supported by growth factors and signals provided by CD4 T helper (Th) lymphocytes, which are commonly subdivided into T helper 1 (Th1) and T helper 2 (Th2) subtypes* (Table 2–1). These effectors are controlled by regulatory T cells (Treg) that are involved in maintaining immune tolerance. Most antigens and vaccines trigger both B and T cell responses, such that there is no rationale in opposing antibody production (‘humoral immunity’) and T cell responses (‘cellular immunity’). In addition, CD4 T cells are required for most antibody responses, while antibodies exert signifi cant infl uences on T cell responses to intracellular pathogens.