**1. Introduction**

Recent research has shown that the erythrocyte contributes in many ways to defense against pathogens. The human erythrocyte is infected by the unicellular eukaryotic parasites *Plasmodium* and *Babesia* and the bacterium *Bartonella* [1]. Bacteria may adhere to glycosylated erythrocyte membrane proteins thereby facilitating phagocytosis by macrophages. Bacteria adhering to the erythrocyte surface may also be killed by oxygen release [2]. The erythrocyte also reacts with morphological changes to infections like Covid-19 or a disease like sepsis. However, the mammalian erythrocyte, having lost its nucleus and organelles, seems to play less a role in the immune system than erythrocytes of fish and birds [3]. The chapter will review aspects of mainly human erythrocytes in immunology and homeostasis with some special attention given to the ongoing Covid-19 pandemic.
