Abstract
This chapter, will provide a first overview on the cells and tissues forming the immune system. We will discuss the general role of immunity in detecting and neutralizing pathogens, in order to reduce the global burden of infectious diseases. In human history of the last thousands of years pathogenic microbes have caused numerous severe pandemics, of which that of SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) is the most recent one. We will distinguish the features and cell types of innate and adaptive immunity and describe their response over time after contact with an antigen. Basically, all cell types of the immune system are created by the process of hematopoiesis, perturbation of which can lead to significant disruption of immune cell production. Finally, we will describe the primary and secondary structures of the immune system, such as bone marrow, thymus, lymph nodes and spleen and the migration of immune cells between them.