This chapter focuses on specific immunity, a complex process involving interactions of the antigens of a pathogen with antigen-receptors and antibodies of a host. These interactions trigger a series of events that either destroy the pathogen or render it harmless. Most of the chapter is devoted to discussions of the functional cells and molecules of specific immunity. During the discussion, the various connections between these cells and molecules are drawn and linked to other types of immune responses. The chapter concludes with a discussion of the ways these responses protect higher animals against viral and bacterial pathogens.