What are the non-immune cells in the bloodstream?
As I mentioned in the section Intro to the Immune System, there are numerous types of cells within the bloodstream that fight off infections in the brain as well as throughout the body. However,there are two other types of cells that are created from the same primitive stem cells which have nothing to do with immunity in particular, however they do serve their own vital roles in the human body. There are two different types of non-immune cells: erythrocytes and platelets, and they are formed from the myeloid stem cells as the Smart Art Graphic below demonstrates:
Erythrocytes are also known as red blood cells (or RBCs for short) and they serve two vital functions in the body: they carry oxygen-rich air to the tissues and expel the waste carbon dioxide that the tissues produce. They are capable of doing this because erythrocytes are composed of haemoglobin (also spelled as hemoglobin, depending on dialect). Haemoglobin is abbreviated as Hb and is composed of four protein parts (two alpha and two beta) which are capable of attaching oxygen. Red blood cells vastly outnumber the amount of white blood cells within the circulation, but they are usually only around a third to half the size. They are red due to the presence of iron in the centre of the haemoglobin molecule.
What do erythocytes look like under the microscope?
In the blood, they appear donut-shaped when mature: they are the only whole cells in the bloodsteam which do not possess nuclei. They actually lose their nuclei during the maturation process: as the picture alongside shows, nucleated (otherwise known as immature or progenitor) RBCs are seen in blood films during trauma as the body tries to compensate for the loss of blood. As you may have observed, they are relatively purple compared to the mature red blood cells and have a round, blotched nucleus. The production of red blood cells is driven by the cytokine erythropoietin.
What function do platelets play within the bloodstream?
Platelets play a vital role in the primary haemostatic process. More information regarding this process can be found in the page Haemostasis. Unlike other types of cells, platelets are actually fragments rather than whole cells: they are fragments of megakaryocytes, which are shown in the picture below.
Megakaryocyte production is driven by the cytokine thrombopoietin. The megakaryocyte goes through the stages below before it is capable of releasing platelets. When the cells become megakaryocytes, they are no longer able to divide.