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16.3B: WBC Function

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    7792
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    Each type of white blood cell (WBC) has a specific function in defending the body against infections.

     

    LEARNING OBJECTIVES

     

    Describe the functions of leukocytes (white blood cells)

     

    KEY TAKEAWAYS

    Key Points

     

    • Leukocyte functions often occur in the bloodstream and may represent either the innate or adaptive immune systems.
    • Innate immune system functions are non-specific and include phagocytosis, inflammation, and degranulation.
    • Adaptive immune system functions are antigen -specific and involve antigen presentation as well as  cell -mediated and humoral -mediated activities.
    • Compared to innate immune system functions, adaptive immune system functions take more time to initiate, but work much faster. They have a memory component to prevent reinfection by the same pathogen.

     

    Key Terms

     

    • macrophage: A white blood cell that phagocytizes necrotic cell debris and foreign material, including viruses, bacteria, and tattoo ink. It presents foreign antigens on MHC II molecules to lymphocytes. Part of the innate immune system.
    • Inflammation: An innate immune system function in response to a pathogen or injury. Chemical mediators cause the blood vessels to dilate and become more permeable, which draws neutrophils to the area.
    • cytotoxic: Any mechanism that can cause the death of a cell (typically without phagocytosis), such as degranulation or cell mediated apoptosis.

    Leukocytes ( white blood cells) provide a number of functions that are primarily related to defending the body from pathogens (foreign invaders). Much leukocyte activity takes place within the bloodstream, but is not restricted to this area. Many leukocytes are able to perform their functions in tissues or organs during normal transport and in response to injury. Leukocyte functions may be classified as either innate or adaptive based on several characteristics.

     

    Innate Immune System Functions

     

    The innate immune system refers to the body’s ability to prevent pathogen entry and destroy pathogens that do enter the body. Its functions are rapid responses that inhibit a pathogen as soon as it is detected in the body. Innate immune system functions involving leukocytes include:

    • Phagocytosis of pathogens. This process is performed primarily by neutrophils, macrophages, and dendritic cells, but most other leukocytes can do it as well. It involves the binding of an Fc receptor to a tail on a pathogen. The pathogen is engulfed by the leukocyte and destroyed with enzymes and free radicals. 
    • Inflammation. This process is performed primarily by mast cells, eosinophils, basophils, and NK cells. When a pathogen is detected or vascular endothelial cells release stress cytokines from injury such as a cut, leukocytes release a variety of inflammatory cytokines such as histamine or TNF-alpha. These cause vasodilation, increase vascular permeability, and promote neutrophil movement to the inflammation site. 
    • Degranulation. This process is performed by granulocytes like neutrophils. When pathogens are encountered, granule-dependent apoptosis (a mechanism of cytotoxicity) may be induced in the pathogen by releasing perforins, granzymes, and proteases from their granules.

    image

     

    Neutrophils Phagocytizing Bacteria: Here, neutrophils are depicted phagocytizing and completely engulfing bacteria.

     

     

    Adaptive Immune System Functions

     

    The adaptive immune system is specific to each pathogen on the basis of antigens, molecular components of pathogens used by leukocytes to recognize that specific pathogen. Compared to the innate immune system, adaptive immune functions work much faster and have a memory component that prevents reinfection by the same pathogen. However, more time typically passes before the adpative immune system is functional. Adaptive immune functions of leukocytes include:

    • Antigen presentation. This process is primarily performed by macrophages and dendritic cells. Following phagocytosis, protein components (antigens) of the pathogen are expressed on leukocyte MHC molecules and presented to naive T cells (and B cells) in the lymph nodes. The T cells will then start the adaptive immune response by rapidly proliferating and differentiating.
    • Cell-mediated activities. This process is performed by T cells. Pathogens that bear the T cell’s antigen are destroyed through cytotoxic -induced apoptosis and protease activity.
    • Humoral activities. This process is performed by B cells, which secrete antigen-specific antibodies. The antibodies bind to pathogens to opsonize (mark) them for phagocytes to engulf, neutralize, or start a complement cascade in which proteins form a membrane attack complex to lyse the pathogen.
    • Memory cell activity. Following antigen presentation, memory B and T cells are created. These rapidly produce new T cells or antibodies if the same pathogen is detected in the future. This prevents that pathogen from reinfecting the organism.