Cells of the immune system/ Cells of Innate and Adaptive immunity
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Both innate and adaptive immunity involves many cells that may act in combinations to eliminate infection or other abnormal conditions. They can be categorized as cells of the innate immune system and cells of the adaptive immune system.
The innate immune system comprises mostly the cells from Myeloid Progenitor. They are Neutrophils, Basophils, Eosinophils, Monocytes, and Mast cells. Neutrophils, Basophils, and Eosinophils are also called polymorphonuclear leukocytes (PMN, PML, or PMNL) because of the varying shape of the nucleus, which is usually lobed into segments. They are characterized by the presence of specific granules in their cytoplasm which contains a combination of chemicals such as antimicrobial agents and hydrolytic enzymes. They release these compounds by a process called degranulation after ingestion of a microorganism.
1. Neutrophils are phagocytic in nature. They have multilobed nucleus and stains with both acid and basic stains. Neutrophils are the most abundant leukocytes and are short-lived (6h to a few days). Neutrophils increase in number in the bloodstream during infection and are responsible for the elevated white blood cell count seen with some infections which is a well-known clinical indication. They are the first cells that act at the site of tissue damage to eliminate pathogens by phagocytosis. They are mainly involved in the elimination of bacteria and fungi.
2. Basophils are non-phagocytic in nature. They have a bilobed or trilobed nucleus. They take up the basic stain methylene blue and appear blue in color. Their life span is a few hours to a few days. Basophils release pharmacologically active substances from their cytoplasmic granules such as histamine which escorts allergens out of the body and blood thinner heparin which prevents clotting. They primarily combat allergic reactions. They also help in the secretion of cytokines involved in the maturation of T-helper cells.
3. Eosinophils are motile phagocytes. They have a bilobed nucleus. They take up the acidic stain eosin and appear red in color. Eosinophils are present in almost all immune responses, most notably allergies. However, they also fight off parasites by secreting membrane-damaging granules.
4. Mast cells have a single-lobed nucleus. They reside in the connective tissue around the blood vessels and mucous membranes. They are typically involved in wound healing and the inflammatory response by releasing chemical mediators, such as histamine and cytokines. Mast cells also have important functions as “soldiers of the immune system” and are early producers of cytokines in response to infection or injury. They are active against many parasites and various allergic tissues. Their lifespan varies from months to years.
5. Monocytes have a kidney-shaped nucleus. They differentiate into macrophages and dendritic cells to elicit an immune response. Monocytes are also known as macrophages after they migrate from the bloodstream and enter the tissue.
• Macrophages are the most efficient phagocytes. They have a single nucleus. Capable of leaving the circulatory system by squeezing through the walls of capillary vessels, they patrol for pathogens or dead cells. Therefore, macrophages are the first line of defense of the innate immune system eliminating a substantial number of targets. Macrophages engulf and process cell debris, pathogens, and cancer cells. Thereby, they also have a role in the activation of T-cells via the mechanism of antigen presentation. Macrophages are involved in wound healing, tissue regeneration, and pro-inflammatory activities. Their lifespan varies from months to years. Macrophages live longer than neutrophils and are especially important for slow-growing or chronic infections.
• Dendritic cells are present in tissues like skin, lungs, and intestines. They can easily identify threats and, act as messengers for the rest of the immune cells. They are indeed considered as a bridge between innate and adaptive immunity. Once they engulf a pathogen by phagocytosis, they act as antigen-presenting cells by presenting the antigen and activate the adaptive immune system by activating B and T lymphocytes.
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Marshall, J.S., Warrington, R., Watson, W. and Kim, H.L., 2018. An introduction to immunology and immunopathology. Allergy, Asthma & Clinical Immunology, 14(2), pp.1-10.
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