English: Overview of the immune system. The immune system contains a complex collection of organs, tissues and vessel systems between which circulate constantly immunocompetent cells of innate and adaptive immunity. These cells communicate with each other via receptors that integrate external signals in the form of soluble mediators or insoluble extracellular ligands or counterreceptors. The binding of a signal induces a transformation of the external information into internal information, a process referred to as a signal transduction inside the cell, and a modification of the properties of the molecular targets involved in a cell signaling pathway. The interaction between different cell types promotes full harmony of their various and vital functions: regulation, homeostasis, immune response, and protection of the host integrity. The alteration of any of these functions causes serious pathological disorders, including autoimmune diseases, cancers, immune deficiencies, and allergic diseases. Finally, a healthy immune system could be described by five main features: (i) self- and non–self-recognition, (ii) non–self-rejection, (iii) modified-self rejection, (iv) tolerance of unmodified self-components, and (v) coherence and integrity.