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White blood cells are a diverse group of cells that originate from different precursor cells. Although the specific function of the white blood cell categories differs widely, all of them exist to defend the organism against non-self agents in the broadest sense. Myeloid white blood cells are produced in the bone marrow. Lymphoid cell development is not restricted to the bone marrow, however, and takes place in primary and secondary lymphoid organs.
The total number of white blood cells in healthy adults ranges between approximately 4-12 × 106 cells/µL, of which lymphocytes and neutrophils make up the majority of cells. Eosinophils, basophils and monocytes are found in far smaller numbers in healthy persons. The absolute count for each white blood cell category in peripheral blood has clinical significance and is more informative than the relative count given in percentages. White blood cell pathologies affecting either the myeloid or lymphoid lineage can be the result of both reactive and non-reactive (malignant) disease. Reactive changes are observed in the course of infectious or inflammatory diseases whereas malignant alterations point to leucaemias, lymphomas and other haematological malignancies.
To distinguish between different diseases related to white blood cells, determining both their number and their exact type and state of maturity is crucial. Automated haematology analysis is a vital component in the diagnostic process and helps identify the presence of disease by providing accurate cell counts and by highlighting conspicuous cell populations. In white blood cell diseases, finding the right diagnosis is complex and needs to consider all the information available from the complete blood count, morphology, immune phenotyping, and other tests.
Neutrophils belong to granulocytes category, which is characterised by the presence of granules in the cytoplasm. They play an important role in immune defense and are the first immune cells to arrive at a site of infection – usually within an hour. This happens through a process known as chemotaxis. Neutrophils can phagocyte other cells such as bacteria which seem harmful to the organism. Yet they will not survive the act themselves. Pus consists mainly of dead neutrophils and digested bacteria.
The absolute and relative neutrophil count can provide some information for diagnosis and monitoring of infections and is also taken into account during chemotherapy. An increased neutrophil count can also be found physiologically in non-pathological situations, e.g. after stress or in smokers.
Lymphocytes defend the organism against infection by distinguishing the body’s own cells from foreign ones. Molecules recognised by the body as foreign are known as antigens. Each lymphocyte is only stimulated by one specific antigen. When lymphocytes recognise this antigen, they produce chemicals to fight it.
There are three main types of lymphocytes: B lymphocytes, T lymphocytes and natural killer cells. Although compared to other white blood cells all lymphocytes are small and round without granules, there is a large variety of different subtypes, and distinguishing between them morphologically is tricky.
Reasons for an increased lymphocyte count include infection or inflammation, as well as certain types of malignancies, especially haematological malignancies. Despite giving an absolute and relative lymphocyte count, several flags on Sysmex analysers can point to suspicious lymphocytes for which, if present, a follow–up test should be performed.
Monocytes have a key role in the immune response. They can quickly move to infection sites and differentiate into macrophages and dendritic cells to provoke an immune response. Cells of the monocyte-macrophage system can engulf foreign particles and break them down to antigens, which they can then present at their surface.
Automated monocyte counts are available as the ratio of monocytes to the total number of white blood cells counted or as an absolute count. An increased monocyte count may be indicative of various disease states, e.g. chronic inflammation or infection, but it may also occur in malignant diseases such as chronic myelomonocytic leukaemia.
Like neutrophils, eosinophils are in the granulocytes category because they are filled with granules containing different enzymes. They can also move and phagocytose (roughly ingest) particles. As they kill parasites by releasing certain cytotoxic enzymes and are involved in hypersensitivity reactions, an increased eosinophil count is most likely associated with parasite infestation or allergy. Eosinophilia can also point to malignant diseases as it is evident in some types of neoplasia.
Basophils are the white blood cells least represented in the peripheral blood and also belong to granulocytes. Similar to eosinophils, an increasing in basophil count often though not always points to allergy or parasitic infections. They function together with mast cells as effector cells in complex processes like chemotaxis or cell adhesion and fulfil an immune modulating role during allergic reactions.