The immune system is composed of several processes that are aimed to defend the body against diseases by detecting and killing disease-causing pathogens and cancer cells (Alberts et al. , 2002). The innate immune system, which is a category of the immune system that is triggered as soon as an infection occurs within the body, is highly capable of distinguishing non-self cells from its own normal cells and will perform specific actions in order to prevent further circulation and proliferation of invading cells.
The process of detecting non-self cells is complex because there are particular pathogens that continuously change their presentation in order to trick the target cell from sensing its presence in the body. Numerous mechanisms have been created to achieve the role of recognition and neutralization of disease-causing pathogens. Examples of these mechanisms include the production of defensins, the employment of the complement system and the development of phagocytosis. More importantly, the immune system further developed to include different types of proteins that could identify and remember pathogens that invade the body.
This immunological process facilitates in prevention of disease and establishment of immunity against persistent infective agents. Neutrophils The neutrophils make up 70% of the white blood cells of the body and comprise an essential component of the immune system. These cells have unique staining features wherein these cells present a neutral pink color under hematoxylin and eosin (H&E) staining. Neutrophils are classified as phagocytes that circulate around the body searching for any event that they could perform they role as defenders of the immune system.
Upon recognition of a bacterial infection in the body, neutrophils exit the circulating bloodstream and migrate to the site of infection through the process of chemotaxis. The role of neutrophils in bacterial infection is generally observed in the production of pus in site of tissue injury and infection. Pus is the white to yellow-colored secretion that is observed in wounds that are infected. The lifespan of a neutrophil lasts from 4 to 10 hours, of which it can be activated at any moment that a bacterial infection has been detected in the body.
Once a neutrophil is activated, it positions itself close to the endothelial lining of the blood vessel and performs capture procedures that are triggered by the protein selectin. Consequently, the neutrophil adhere to its new location where the inflammation is occurring. An activated neutrophil commonly last for 1 to 2 days. Research has suggested that the short life span of neutrophils is an adaptation that prevents further distribution of the pathogens within the body after the neutrophils engulf the bacterial cell.
This short existence of neutrophils may also possibly prevent further damage that may occur to the body during the inflammatory stages of infection. The process of phagocytosis that is inherent among neutrophils involves the internalization and killing of bacterial cells. The internalization process results in the generation of a phagosome which is a subcellular structure that serves as a compartment for the administration of hydrolytic enzymes that would kill the invading bacterial cells (Finlay and McFadden, 2006).
Reactive oxygen such as superoxide dismutase is also produced in the phagosome hence the process of killing bacteria through this route has been termed as respiratory burst. Superoxide dismutase is activated by the enzyme catalase to generate hydrogen peroxide. This is further converted into hypochlorous acid will is deleterious to bacterial cells. Neutrophils have the capability of producing three kinds of granules. Azurophilic granules are generally composed of myeloperoxidases, defensins, elastases and cathepsin G, which are all involved in the degradation of a bacterial cell.
Secondary granules consist of lactoferrin and cathelicidin, while tertiary granules are made up of cathepsin and gelatinase. Similar to neutrophils, macrophages are phagocytic white blood cells that arise from monocytes. Just like neutrophils, macrophages perform non-specific defense responses to infections that occur within the body. Macrophages have the responsibility of engulfing and digestion of fragments of cells and pathogens and this process activates lymphocytes and other components of the immune systems to react to the presence of pathogens in the body.
The life cycle of a neutrophils starts with the entry of a monocyte into the endothelial lining of a blood vessel. This leukocyte adhesion process then further progresses through a battery of modifications which eventually gives rise to a differentiated macrophage. Monocytes have the intrinsic ability to sense cellular or tissue injury through the process of chemotaxis, which is associated with the detection of stimuli such as damaged cells, histamine secreted by mast cells and basophils and cytokines generated by macrophages. Opposite to neutrophils, macrophages are known to circulate for several months to years.
Macrophages Macrophages perform their function in the immune system through three steps. Firstly, a macrophage engulfs a bacterial cell or any cellular debris through the process of phagocytosis. This process results in the production of the subcellular component phagosome inside the macrophage. Secondly, the phagosome fuses with a lysosome which is another subcellular component that contains degradative enzymes. The resulting structure, the phagolysosome, thus serves as a site of degradation of the engulfed material within the macrophage.
Thirdly, the materials that have been degraded are either extruded out of the macrophage or are assimilated by the macrophage. The main responsibility of the macrophage is to eliminate any cellular debris and other unimportant materials that are circulating in the body. Such role is essential in the early stage of bacterial inflammation because it serves as the first line of defense of the innate immune system of the body. Macrophages are often positioned in strategic places that are commonly prone to infection and inflammation. These places include the lungs, liver, tissues of the nervous system, bone and spleen.
Each wandering macrophage thus engulfs a pathogen which is further degraded by the enzymes that are present in the lysosome. Components of the engulfed pathogen are also presented to the cells of the immune system in order to recognize and remember the features of the invading cells, as well as to generate antigen-specific antibodies that will trigger binding. Antigen-specific antibodies are produced by helper T cells while the presentation is performed by the major histocompatibility complex class II (MHC Class II) cells (Janeway et al. , 2001).
Once the antibodies are generated, these in turn attach to the antigen and the resulting complex is detected by the circulating macrophages and will consequently be engulfed and degraded. B-cells are also a component of the innate immune system and these cells also generate antibodies that facilitate opsonization or the production of a complex that will help in the identification and phagocytosis of invading pathogenic bacteria. Different types of macrophages exist in specific locations of the body. In the lungs, the alveolar macrophages detect any invading pathogens.
In the connective tissue, there are histiocytes that are strategically located in that area. Kupffer cells are present in the liver while microglial cells wander around in the neural tissues. Osteoclasts are present in the bone and the spleen contains sinusoidal lining cells that act as macrophages. Conclusions The immune system is composed of several processes that are aimed to defend the body against diseases by detecting and killing disease-causing pathogens and cancer cells. Several mechanisms of recognition and neutralization of disease-causing pathogens have been employed by our innate immune system to combating bacterial infections.
The process of detecting non-self cells is complex because there are particular pathogens that continuously change their presentation in order to trick the target cell from sensing its presence in the body. It is also of prime importance that the evolution of bacterial pathogens be investigated because this would help in combating future infections that may involve newer, stronger and drug-resistant strains. In addition, resources for antimicrobial proteins such as vitamin D from sunlight would also be worthwhile analyzing.
Several anecdotal reports and research results that claim that vitamin D triggers the immune system to combat bacterial infections by producing defensin proteins. References Alberts B, Johnson A, Lewis J, Raff M, Roberts K and Walters P (2002): Molecular Biology of the Cell, 4th ed. New York and London: Garland Science. Finlay B and McFadden G (2006): Anti-immunology: evasion of the host immune system by bacterial and viral pathogens. Cell 124(4):767-82. Janeway C, Travers P, Walport M and Shlomchik M (2001): Immunobiology, 5th ed. New York and London: Garland Science.