ABSTRACT
INTRODUCTION The primary function of the immune system is to protect the host from infectious agents. Environmental pathogens threaten the host with a wide spectrum of pathologic mechanisms, and the immune response uses a complex array of protective mechanisms to control and usually eliminate infectious organisms. All of these mechanisms rely on detecting structural features of the pathogens that mark them as distinct from host cells. Such hostpathogen discrimination is essential to permit the host to eliminate the pathogen without excessive damage to its own tissues. Host mechanisms for recognition of microbial structures can be broadly distinguished in two general classes: innate immune responses encoded by genes in the host germline recognizing molecular patterns shared by many microbes but absent in the mammalian host; and adaptive immune responses encoded by gene elements that somatically rearrange to assemble antigenbinding molecules with exquisite specificity for unique microbial and environmental structures.
