Principles of the immune system: Players and organization

Abstract

All living organisms are in constant exchange with the environment: its climate and physical conditions, chemicals, radiation, and last but not least other organisms. Among these, pathogenic microbes are a constant threat to health. Consequently, all organisms have developed strategies to prevent damage by bacterial, viral, and fungal pathogens. In vertebrates a highly complex defense mechanism has evolved, which goes far beyond "eat-and-destroy-the-bugs" strategies of evolutionary older organisms like prokaryotes or invertebrates. This mechanism is called the immune system. The immune system is different from other organs. It is distributed across the entire body and comprises organ structures, cells, and molecules such as antibodies and cytokines. It has a vast array of functionally different cells (T and B lymphocytes, macrophages, neutrophils, mast cells, to name just a few). While some cells remain tissue bound, others can move around the body and shuttle between the blood and lymph vessels and tissues. The immune system comprises lymphoid organs such as bone marrow, thymus, lymph nodes, or the spleen, which all form developmental niches for immune and blood cell development (hematopoiesis) and act as contact centers for interaction of immune cells. Indeed, the immune system is constantly renewing itself from hematopoietic stem cells, and the highly organized differentiation of cells is an important functional feature of immune responses. The immune system is unique in its memory: it will fight pathogens better, faster, and more vigorous on a second encounter. Finally, the immune system is a paradigm of checks and balances. In order to prevent attacking and damaging its own body or overreacting to harmless environmental cues, immune responses not only entail immune activation but also spend much effort on immunosuppression. Adverse immune reactions occur when either the discernment of antigens as harmless or harmful fails or when the immune system attacks proteins of its own body as antigens. Finally, chemicals, drugs, stress, or certain environmental conditions can suppress the immune system with severe health consequences. Immunotoxicology is the discipline which deals with unwanted, adverse immune responses caused by the (chemical and physical) environment. In this chapter, I want to introduce major players of the immune system and major current concepts of its operation strategies, with a special emphasis on interactions with the environment. This shall give a framework for a better understanding of the chapters which deal with particular situations of the environment-immune system interaction.