The therapy, mechanism as well as model of immunodeficiency are described in this all-inclusive book. It sheds light on one of the most common medical issues, which, unfortunately is still being researched on. Various medical issues are connected in a straight or indirect way to a particular alteration of the immune system. Therefore, they result in varied pathologic circumstances. Human defense mechanism is complex and different, but is many times defeated by infections, tumor processes, organ, tissue and cell transplantation, allergy, autoimmune processes, as well as various impacts by the environment. This book, jotted down by globally recognized scientists, discusses in detail the questions of what, why, how, when and where the immune system falls weak, what scientific results are exhibited and what are the techniques to cure and prevent immunodeficiency cases. This book will be beneficial to the learners and practitioners of medicine and biology, and will also be knowledgeable to immunologists, microbiologists, virologists, chemotherapists, oncologists, hematologists, transplantologists and pathologists.