Traditional medical science has kept the mind separate from the body. Now people realize that the psyche and the soma are constantly interacting. And psychoneuroimmunology is the new evolved science that describes such kind of interactions. Many studies show that psychological stress (acute, brief naturalistic and chronic) has immune modulatory activity. In this book a typical psycho-neuro-endocrine-immune network has been shown. From this network Corticotrophin Releasing Factor (CRF), Adrenocorticotrophic Hormone (ACTH), Glucocorticoids (GC), β-endorphin and Met-enkephalin are found as important endocrine components, and T cells, B cells, monocytes/macrophages, Natural Killer(NK) cells and their cytokines that is Tumor Necrosis Factor-α (TNF-α), Interferon Gamma (IFN-γ) and interleukins such as IL-1, IL-2, IL-4, IL-6, IL-10, IL-12 etc. are found as important immune components. Finally, it has been shown that, acute, brief naturalistic and chronic stress have different immune modulatory activities which are harmful to one's homeostasis, and relaxation can help to maintain that homeostasis.