Book excerpt: This book gives insight into the mechanism of the immune system and the influence of the environment on earth. Further, the book explains the changes that occur in our immune system in the absence of gravity and their fundamental consequences. Several limiting factors for human health and performance in microgravity have been clearly identified as an unacceptable risk for long-term and interplanetary flights. Serious concerns arose whether spaceflight-associated immune system dysfunction ultimately precludes the expansion of human presence beyond Earth's orbit. The immune system has undergone many evolutionary steps to cope with a new and changing environment, but `space` has not been evolutionary experienced before. Through endocrine orchestration of cell functions, cell to cell communications and intracellular mechanisms the human body and his immune system have an enormous capacity to adapt and react to altered environmental conditions. Thus, the special sensitivity to altered gravity renders the immune system an ideal biological model system to understand if and how gravity on Earth is required for the normal function of cells and cellular networks. It is one of the most fundamental challenges to find out, if our organism and our cellular machinery are able to live and to adequately perform without the gravity field of Earth. The book is written for immunologists and researchers in human physiology under normal and stressfull conditions.