Erythrocytes

Abstract The erythrocyte is a highly specialized cell present in the blood designed for the transport of oxygen from the lungs to the body tissues and for the removal and transport of carbon dioxide from the tissues to the lungs. In primates, erythrocytes also clear the blood of complement‐bearing immune complexes and pathogens and exert anti‐inflammatory functions. The blood of an average adult contains approximately 25 million millions of erythrocytes. Maintaining this number of cells constant over time is a major challenge and requires production of as many as 210 billions of new erythrocytes per day (i.e. 24 millions per second). This production is tightly regulated by microenvironmental cues (growth factors) and by proteins present inside the cells (transcription factors). This regulation ensures that erythrocyte production is appropriately increased in response to sudden blood loss. Key Concepts: Erythrocytes are blood cells responsible for oxygen delivery to all the tissues of the body. Erythrocytes are produced by hematopoietic stem cells present in the bone marrow. Before birth, erythrocytes are derived from stem cell populations present in the yolk sac of the embryo and in the liver of the fetus. Erythropoietin, a cytokine produced by the kidney, is primarily responsible for the regulation of erythrocyte production. The bone marrow also contains several cytokines, which positively or negatively regulate erythrocyte production. Erythrocytes exert their function by expressing on their surface and in their cytoplasm a number of erythroid‐specific proteins. A series of DNA‐binding proteins (the erythroid transcription factors) assure that erythrocytes express adequate amounts of functional proteins. The erythrocyte membrane is plastic and resistant to shear stress allowing the cells to pass through the microvasculature to deliver oxygen. The oxygen delivery function of the erythrocyte is performed by haemoglobin, a cytoplasmic protein whose structure is suited for oxygen transport/exchange. The flexibility/deformability of the erythrocyte membrane is assured by a network between the proteins embedded in its lipid bilayer and the cell cytoskeleton. Blood group antigens are epitopes present on the tertiary (glycosyl residues) or primary (amino‐acid sequence) structure of proteins present on the erythrocyte membrane.