This book will look at some of the main features of the immune system and how they defend the body from pathogens and other invaders. It will also look at problems that can arise with the immune system.Review the book according to its contents definition of the immune system, its parts, its mechanism of action, and the regulation of its secretions, with a focus on their chemical composition and mechanism of action in order to be accurate in the information.Finally, I hope that this scientific effort will achieve the desired benefit for science students, professors, researchers, and specialists in this field.