"The immune system could be described as a sensory organ much like vision, hearing and touch. Instead of recognising light, sound waves or large objects, the immune system recognises molecular shapes - only a few amino acids in length. Those shapes lacking a self identification tag may be recognised as foreign, triggering a series of events that eventuate in an immune response. This module discusses the various ways in which the body protects itself against infection and how vaccination works with these processes. The description that follows is simplified; however there are many excellent immunology textbooks and websites are available to provide additional detail and are recommended at the end of this module. Keep in mind that the immune system is like an octopus - therefore difficult to cover in a linear page-by-page fashion."
The link is a nice, clearly written summary of the human immune system.