CK-12 Foundation
Ck 12: Biology: Immune System Part I Study Guide
This study guide on the immune system covers key terminology and the different lines of defense the human body has to protect itself against pathogens. It is available for download with free registration.
Other
Clermont College: Immune System
This complete resource provides a thorough discussion of the immune system. Topics include the non-specific defense mechanism, specific defense mechanism, humoral immune system, cell-mediated immune system, and more.