Host immunobiology and vaccine development.

As the rules of immunoregulation become clearer, the design of vaccines and adjuvants is becoming more scientific. To understand these rules, the interactions between three kinds of cells need to be grasped. Antigen-presenting cells (APCs) initiate the immunoglobulin cascade. The most important of these are dendritic cells, which must first capture antigen, a process aided by particulate matter, the presence of natural or acquired antibodies, or the capacity to activate complement. Then T cells become activated through conjoint action of processed antigenic peptides and APC surface and secreted molecules. T cells mediate inflammation, develop cytotoxic capacity, and help in antibody formation. Whether cells of type 1 or type 2 predominate influences the direction of both cellular and humoral responses. B cells are then activated, leading to antibody formation and often to better antigen presentation. Both T and B cell memory, embedded in long-lived lymphocyte populations, aid heightened immune reactivity when the antigen is re-encountered. The best vaccines stimulate strong memory.