Natural antibodies target virus-antibody complexes to organized lymphoid tissue.

Natural antibodies (NA) specific for infectious pathogens are found at low titer (usually <1:40) in the serum of healthy, non-immunized, individuals. Therefore, NA are part of the first line of defence against blood borne microorganisms. They directly neutralize viral infections or lyse pathogens by activating the complement cascade. In addition, recent studies highlighted their role in the pooling of infectious pathogens and other antigens to the spleen. This prevents infection of vital target organs and enhances the induction of adaptive immune responses. Specific T and B-cell responses are exclusively induced in highly organized secondary lymphoid organs including lymph nodes and the spleen. As a consequence, mice with disrupted microorganisation of lymphoid organs have defective adaptive immunity. In addition, some pathogens including lymphocytic choriomeningitis virus (LCMV), Leishmania and HIV developed strategies to destroy the splenic architecture in order to induce an acquired immunosuppression and to establish persistent infection. NA antibodies enhance early neutralizing antibodies in the absence of T help mainly by targeting antigen to the splenic marginal zone. In addition, by activating the complement cascade, NA enhance T cell and T-cell dependent B-cell responses. Therefore, natural antibodies are an important link between innate and adaptive immunity.