Brief introduction to the basic scientific principles of hematopoietic stem cell transplantation (HSCT)

The primary function of the immune system is to provide essential defense mechanisms against all foreign pathogens. The immune system has evolved in such a way that different immune responses are optimized to recognize and then eliminate or contain different types of foreign antigens which are expressed or secreted by foreign pathogens. It provides not only efficient and effective killing of microbes/pathogens via innate immunity but also specific long-lasting immunity against a particular microbe/pathogen to be triggered if the foreign microbe’s antigen is encountered in the future via adaptive immune responses. Immunologic mechanisms are intimately involved in engraftment, engraftment rejection, graft versus host disease, and graft versus malignancy effect. In addition, immunologic tolerance is key for allogeneic immune reconstitution post-hematopoietic stem cell transplantation (HSCT). Because of a better understanding of the immune system and its different immune properties and responses, physicians and researchers have been able to perform successfully and safely HSCT in humans. While many of the concepts of basic immunology and transplant biology are intertwined into other chapters of this book, this chapter focuses on providing the fundamental principles of basic immunology and transplant biology, including the development of the components of the immune system (i.e., hematopoiesis), the molecules, cells, tissues, and organs that make up the immune system as well as their structural and functional organization and the types of immune responses along with their cardinal features. Key concepts related to HSCT including antigen presentation, alloreactivity, and tolerance and how these processes relate to HSCT will be described in brief. Firstly, though, this chapter begins with the definitions of some key terms and concepts related to basic immunology and transplant biology in order to establish the “vocabulary” of the immune system.