16.3B: WBC Function

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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 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.

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