The study of the molecular and cellular components that comprise the immune system, including their function and interaction, is the central science of immunology.     
The immune system has been divided into a more primitive innate immune system and,in vertebrates, an acquired or adaptive immune system

The field concerning the interactions among cells and molecules of the immune system,andhowsuchinteractionscontribute to therecognition and elimination of pathogens. Humans (and vertebrates in general) possess a range of nonspecific mechanical and biochemicaldefenses against routinely encountered bacteria, parasites, viruses, and fungi.The skin, for example, is an effective physical barrier toinfection. Basic chemical defenses are also present in blood, saliva, and tears, and on mucous membranes.
True protection stems from thehost's ability to mount responses targeted to specific organisms, and to retain a form of “memory” 
that results in a rapid, efficient response toa given organism upon a repeat encounter. This more formal sense of immunity, termed 
adaptive immunity, depends upon the coordinatedactivities of cells and molecules of the immune system.