 |
| (© Eric Reits, with changes by Claire Boog and Wilma Witkamp) |
All blood cells are derived from the bone marrow. They are all the progeny of the common pluripotent haematopoietic stem cell. The haematopoietic stem cell then differentiates into a myeloid progenitor and lymphoid progenitor. The myeloid progenitor then further differentiates into blood cells, mast cells and monocytes. There are three types of blood cells:
- Erythrocytes: commonly known as red blood cells. They carry nutrients and oxygen around the body to provide for other cells.
- Leukocytes: commonly known as white blood cells. They are the main defenders of the body as is part of the immune system.
- Thrombocytes: commonly known as platelets. They form clots in sites of trauma to prevent blood loss while the body repairs the tissue.
Leukocytes are white blood cells and are highly important in immune response. There are many types of white blood cells each with their own function.
Neutrophil: they are very common and important in the innate immune response. Neutrophils are categorized as a granulocyte and are characterized by their multi-lobed nucleus that is observed when stained. They are the first cells to arrive at sites of inflammation and start phagocytosing foreign pathogens. They use the granules to degrade foreign material engulfed. Neutrophils are known to have myeloperoxidase as part of their granules.
Eosinophil: they are not so common leukocytes and are involved in parasite defence and allergic response. They are characterized by their bi-lobed nucleus when stained.
Basophil: they are important in allergic inflammatory responses and in the release of histamine.
Mast cell: mast cells are implicated in allergic inflammatory response as well as mast cell degranulation including the release of histamine and heparin. This usually results in severe allergic responses.
Monocyte: monocytes are blood cells that circulate in the blood stream till they move into a tissue and differentiate further into two more important immune cells:
- Dendritic Cells (DC): these cells are easily characterized due to their arm-like dendrites extended from the cell. They tend to hang around in tissues and also the lymphatics. DCs are important antigen presenters as part of the adaptive immune system. They are characterized by the marker CD11c.
- Macrophages: are also resident in tissues. Their main task is to phagocytose foreign pathogens and anything else that is not host.
The haematopoietic stem cell also differentiates into the lymphoid lineage and this leads to the production of lymphocytes. These cells are primarily located in the lymphatic system. The lymphoid progenitor leads to the production of two main types of lymphoid cells:
B-lymphocyte: acquire their name from the bursa of Fabricius in birds. They are matured in the bone marrow. They are important cells in the humoral immune system and the primary producers of antibodies. B-cells are initially called mature naïve cells, which means they are fully functional, but not yet ready or activated. They work together with other lymphocytes to be activated and start producing antibodies. B-cells are the third type of professional APCs. B-cells are also important for long term adaptive immunity.
T-lymphocyte: are also derived from the bone marrow. However, T-Cells travel to the thymus where they mature into CD4+ or CD8+ cells. The CD4+ T-cells usually become helper T-cells, which often work with B-cells in the adaptive immune system. The CD8+ T-cells are essentially killer cells that look for signals to kill other cells.
