Immune responses in the absence of costimulation: viruses know the trick.

Costimulatory molecules are crucial for the induction of immune responses after immunization with purified proteins or peptides. However, some viruses and other pathogens are able to induce protective immunity in the absence of such molecules. This review argues that patterns recognized by both the specific and the innate immune system, together with a high and sustained Ag-load, are responsible for these surprisingly efficient immune responses triggered by pathogens.