The differential leukocyte count provides the relative per centage of each class of white blood cells. In addition to these values, hematological instruments generally report the absolute values of the various classes. The differential count also allows the detection of cells normally not present in the circulation, such as immature elements, or abnormal populations, such as, for example, leukemic cells. The differential count is obtained mainly by two methods: microscopic and automated. Leukocytes are classified into granulocytes, lymphocytes, and monocytes.

Granulocytes owe their name to the presence of distinct cytoplasmic granulations that define three classes: neutrophils (also called polymorphonuclear), eosinophils, and basophils. Neutrophil granulocytes are mature cells that originate from an initial morphologically identifiable precursor in the marrow, the myeloblast. As maturation progresses, the myeloblast becomes first a promyelocyte and then a myelocyte. In these proliferative stages, the number of cells increases geometrically. The metamyelocyte, the next cell, no longer has mitosis and then transforms into a granulocyte with a banded nucleus (band granulocyte). At this stage, the cell is released into the circulation, where it completes its maturation or enters medullary compartments to mature, from which it is later released into the circulation. About half of the intravascular polymorphonucleates circulate, maintaining a dynamic equilibrium with the other half, which is “marginalized” on the vascular endothelium. Therefore, only the circulating neutrophils are considered in the differential count. The half-life of mature circulating neutrophils is about 7 h. They irreversibly cross the vascular endothelium, migrate into the tissues to perform antibacterial surveillance, and die after 1 or 2 days. Eosinophilic and basophilic granulocytes have a similar development. After their release from the bone marrow, eosinophils rapidly leave the intravascular compartment and migrate into tissues, especially the gastro intestinal tract, lung, and skin, and never return to circulation. The function of these cells is probably to defend against multicellular parasites and to control the mechanisms associated with allergies and asthma, as well as inflammatory phenomena. Basophil granulocytes constitute about 1–2% of circulating leukocytes. Their physiological role is not yet completely clarified. They are present in inflammatory reactions during the immune response, as well as in allergic manifestations. Their granules contain heparin and histamine.

Lymphocytes and monocyte-macrophages are known as mononuclear leukocytes. Both play an important role in cellular and humoral immunity. These cells can get in and out of circulation, maintaining their function, and can spend a long time in tissues or lymph nodes.

Lymphocytes are the key immune cells for cellular and humoral immunity. In the blood, they represent 20–45% of leukocytes. Morphologically indistinguishable, they belong to two distinct systems called T (thymus) and B (bursa or bone marrow), having different functions. B lymphocytes are responsible for the synthesis of antibodies. When a lymphocyte of the B system (humoral system) is properly stimulated, it first proliferates and then turns into a plasma cell, the effector part of the immune response. Each B lymphocyte can synthesize only one type of antibody. The T lymphocyte system constitutes the cellular immune system and regulates the entire immune system. Different subsets of T cells can be identified by monoclonal antibodies specific against different membrane antigens. For example, T helper lymphocytes promote the function of B lymphocytes, while T suppressor lymphocytes inhibit them. Some T cells are responsible for cell-mediated cytotoxicity; natural killer (NK) lymphocytes are responsible for the nonspecific lysis of certain cells, for example, cancer cells. In peripheral blood, about 15–25% of lymphocytes are B cells, and about 40–75% are T cells.

Monocyte-macrophages originate in the bone marrow, from where they are rapidly released into the circulation, where they account for ∼5% of leukocytes. Once they migrate into tissues, they become macrophages. Monocyte- macrophages play phagocytic functions on bacteria and particulate material, participate in inflammatory reactions, and are important to the immune system as they process antigenic material and communicate related information to T lymphocytes through a process of cell–cell interaction. Monocytes can also secrete interleukins, which boost B and T lymphocytes. Additionally, they participate in fibrinolysis by secreting plasminogen activators.

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مواضيع ذات صلة

Sideropenic and Assimilated Anemia

Automated Differential Count

Reticulocytes and related Indices

lactose tolerance test (Hydrogen breath test)

Lactoferrin

lactic dehydrogenase (LDH, Lactate dehydrogenase)

lactic acid (Lactate)

ischemia-modified albumin (IMA)

iron level and total iron-binding capacity (Fe and TIBC, Transferrin saturation, Transferrin)

Platelet Indices and Platelet Count

Red Cell Distribution

Flow Cytometry