Author: Prof. Dr. med. Peter Altmeyer

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Last updated on: 29.10.2020

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Leukocytes (leukos = white; kytos = vessel, cell) are cells of the body that can be detected in the blood, bone marrow, lymphatic organs and other body tissues. Leukocytes fulfil special tasks in the defence against pathogens and structures foreign to the body. They belong to the immune system and are part of the specific and unspecific immune defence (see immunity). The individual subgroups of leukocytes take on various tasks within the immune system such as phagocytosis, marking of antigens, up to the fight against the body's own and foreign cells and cancer cells.

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The following subgroups of leukocytes are distinguished:

  • Granulocytes:
    • Neutrophil granulocytes, monocytes, macrophages and dendritic cells are capable of phagocytosis as part of the unspecific defence (phagocytes or macrophages).
    • Eosinophil granulocytes: defence against parasites, participation in allergic reactions, also in tumorous processes.
    • Basophilic granulocytes: defence against parasites, triggering allergic reactions, inflammatory reactions, itching.
    • Mast cells: after activation, they release substances that affect the permeability of blood vessels.
  • B lymphocytes:
    • B lymphocytes produce special antibodies directed against certain pathogens or harmful substances after appropriate stimulation. They thus belong to the specific defence. Antigen-specific B cells with specific monomeric receptor molecules of the IgM class are already available on first contact with an antigen. An antigenic stimulus leads to the expression of allergen-specific antibodies of certain immunoglobulin classes on the cell membrane (IgM, or change from IgM to the immunoglobulin classes IgD, IgG, IgA or IgE).
    • B-memory cells (memory cells): During prolonged antigen stimulation, B-memory cells are formed which can react more quickly to specific antigens upon re-exposure.
    • Plasma cells (specialization in antibody production).
  • T-lymphocytes: they serve to coordinate specific and non-specific defence.
    • T4 helper cells: activation of plasma cells and killer cells. They recognise antigens on the antigen presenting cells in conjunction with MHC class II molecules. Furthermore, they induce cell-mediated and humoral immune response, stimulate cytokine release, e.g. of IL-2, IFN-γ, TNF-α, IL-5, IL-17.
    • T8 suppressor cells: recognition of endogenous peptide antigens (in conjunction with MHC class I molecules). Two signals are necessary for activation: cytokine stimulation by T4 helper cells and specific recognition of the peptide antigen/MCH class I complex expressed on the target cell.
    • T-regulator cells (Treg): inhibition of the immune response; inhibition of the function of B cells and other T cells).
    • T-killer cells (cytotoxic T-cells): recognition and destruction of virus-infected body cells and tumour cells by reacting to certain antigens of the infected cells.
  • Natural killer cells (NK cells): they attack non-specific cells that are infected by viruses or tumours.

General information
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Leukocytes are involved in all inflammatory processes of the organism. They are able to maintain, modulate or terminate an inflammation by means of released mediators (see also cytokines; see also leukotrienes). Leukocytes continue to play a formative role in all autoimmune diseases. The size of leukocytes varies between 7 µm for lymphocytes and 20 µm for monocytes. Erythrocytes are about 7.5 µm in size. The life span of the cells ranges from a few days to several months. Certain leukocytes are amoeboid mobile and can actively migrate from the blood into the various cell tissues.

Standard value: The standard value of B leukocytes is 2 - 3 J. 6,000 - 17,000/µl; 4 - 12 years. 5.000 - 13.000/µl; adults 3.800 - 10.500/µl. In CSF the leucocyte count is 0 to 5 /µl

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Normal human blood smear under the light microscope:
  • Adults: 4.000-10.000 /µl
  • School children: 5.000-15.000 /µl
  • Infants: 6.000-17 500 /µl
  • Newborns: 9.000-30.000 /µl.
A number of leukocytes per volume exceeding the norm is called leukocytosis. If the number of leukocytes per volume falls below the standard values, this is called leukopenia. An increase in the number of eosinophilic leukocytes in the blood is called eosinophilia.


Last updated on: 29.10.2020