In this chapter, the composition of blood is described. This includes the structure and function of red blood cells as carriers of oxygen and carbon dioxide, as well as other activities important in maintaining the stability of blood. The white blood cells comprise several different populations with diverse structures and functions that enable them to do their work both within and outside the blood vessels. The way of distinguishing the different cells from one another is described in detail, especially through their ‘Cluster Determinants’. In the same chapter, the composition and the functions of the liquid part of blood, Plasma, are described, as well as its interactions with the extravascular and interstitial fluids in our bodies. Furthermore, the plasma proteins are listed, and their function is described in detail concerning the field of immunity. The chapter ends with the description of two related systems—complement and inflammation—that are very important not only to fight foreign pathogens like bacteria and viruses but also in maintaining the body in a steady state.