Vaccines induce the activation and differentiation of antigen specific-T and B lymphocytes, and the development of humoral and cellular effector mechanisms responsible for the destruction of pathogenic agents. The characteristics of the immune response depend on the nature of the antigen, the route of administration of the vaccine and the genotype of the vaccinated individual. Antigen recognition and B and T lymphocyte interactions take place in secondary lymphoid organs. Antigen-stimulated lymph nodes exhibit dramatic changes involving the lymphoid follicles where major events such as clonal selection, differentiation and maturation of B- and T-cell precursors occur. These events include various cellular and lymphokine signals termed cellular cooperation. Different effector mechanisms participate in antigen elimination.