Protective immunity to intracellular parasites--a focus for vaccine research and a challenge to immunization programmes.

Protective immunity to intracellular parasites is mediated by T cells. This response is complex, requiring activation of several subgroups of T cells and production of a balanced blend of cytokines to obtain protection rather than delayed hypersensitivity leading to tissue damage. To obtain additional, detailed information, on the kind, proper localization and specificity of the required immune response represents a major challenge in current vaccine research. Insufficient knowledge of these features is the major constraint for development as illustrated in tuberculosis and related mycobacterial infections.