Introduction
- In the intricate world of the immune system, natural immunity plays a crucial role in defending our bodies against pathogens. One aspect of natural immunity involves the participation of cellular factors that work together to protect us. In this article, we will explore two key cellular factors: phagocytosis and natural killer cells (NK cells). Understanding their functions and mechanisms will shed light on the remarkable defense system that our bodies possess.
Phagocytosis
Definition and Historical Background
- Phagocytosis, derived from the Greek words “phagia” meaning “eating” and “cytos” meaning “cell,” is a remarkable cellular process that engulfs foreign particles.
- The discovery of phagocytic cells can be attributed to the work of Metchnikoff in 1838. These cells, known as phagocytes, can be divided into two types: microphages and macrophages.
Types of Phagocytic Cells
- Microphages and macrophages are the primary professional phagocytes. While other cells can occasionally perform phagocytosis, these two types dominate the process.
Microphages: Polymorphonuclear Leukocytes (PMN)
- Microphages, consisting of neutrophils, eosinophils, and basophils, are polymorphonuclear leukocytes present in the bloodstream.
- These cells, characterized by multilobed nuclei, are the most abundant white cells in circulation. Although they have a short lifespan and do not divide, they play a crucial role in engulfing foreign particles.
Macrophages: Mononuclear Phagocytic System
- Macrophages, part of the mononuclear phagocytic system (originally termed the “reticulo-endothelial system”), arise from promonocytes in the bone marrow.
- These monocytes, representing 2-10% of total white blood corpuscles, circulate in the blood for a brief period before migrating into the tissues.
- Once settled, they differentiate into macrophages, which can be free or fixed in specific tissues. Macrophages, with their extended lifespan, are primarily involved in engulfing bacteria, viruses, and protozoans.
Process of Phagocytosis
- Phagocytosis occurs through several stages: chemotaxis, attachment, ingestion, intracellular killing, and digestion.
- Chemotaxis is the movement of phagocytes toward the site of infection or inflammation in response to chemotactic factors produced by foreign particles or damaged tissues.
- Opsonins, such as antibodies or activated complement factors, aid in the attachment of phagocytes to foreign particles. This attachment initiates the process of ingestion.
- During ingestion, the cell membrane of phagocytes produces pseudopodia that surround the particle, forming a vacuole called a phagosome.
- The phagosome then fuses with the lysosome, resulting in the formation of a “phagolysosome” or “secondary lysosome.”
- Intracellular killing involves the action of antimicrobial substances produced by the lysosomes.
- These substances, including lysozyme, hydrogen peroxide, and myeloperoxide, exhibit antibacterial properties.
- Polymorphs (PMN) also contribute to the antibacterial action with their cationic proteins and lactoferritin.
- The final stage is the digestion of killed organisms and cells by the hydrolytic enzymes within the lysosomes. The products of digestion are subsequently eliminated from the phagocyte.
- Phagocytes can also cause cell injury to neoplastic cells and other cells by direct contact without ingesting them. This type of injury is known as “contactual” cell injury.
Natural Killer Cells (NK Cells)
Unique Characteristics
- Natural killer cells, also referred to as NK cells, are non-phagocytic lymphocytes characterized by large granules.
- These cells possess natural cytotoxicity, enabling them to kill a wide range of tumor cells and cells infected with viruses without the need for antigen stimulation.
- They can independently identify and eliminate tumor cells and virus-infected cells, earning them the name “natural killer cells.”
Development and Maturation
- NK cells originate from lymphoid progenitors alongside B and T lymphocytes.
- They undergo differentiation and maturation in various organs, including the bone marrow, lymph nodes, spleen, tonsils, and thymus.
- Once mature, they enter the circulation, functioning as components of both the innate and adaptive immune systems.
Functions of NK Cells
- NK cells perform several vital functions in the immune system:
- Apoptosis: They induce the death of viruses hidden inside infected cells.
- Lysis of virus-infected cells: NK cells directly destroy cells infected with viruses.
- Lysis of tumor cells: They recognize and eliminate tumor cells.
- NK cells respond to cytokines secreted by virus-infected cells, leading to their activation. Once activated, NK cells release gamma interferons (IFN) that further activate macrophages, promoting phagocytosis and the lysis of virus-infected cells.
- NK cells selectively kill body cells possessing low levels of class I MHC molecules.
- NK cells specifically target and kill tumor cells.
- Activation of NK cells triggers the release of chemical substances, such as perforin or cytolysin, into the space between the cells. Perforin causes lysis of the virus-infected cell, leading to the cell’s demise.
Conclusion
- In the realm of natural immunity, cellular factors play a vital role in defending our bodies against pathogens. Phagocytosis, carried out by microphages and macrophages, involves a series of stages that ultimately result in the elimination of foreign particles. Natural killer cells, on the other hand, exhibit natural cytotoxicity and independently recognize and eliminate tumor cells and virus-infected cells. Together, these cellular factors contribute to the intricate defense system that keeps us healthy.
FAQs (Frequently Asked Questions)
- Are phagocytes only present in the bloodstream?
No, while phagocytes like microphages (neutrophils, eosinophils, and basophils) dominate the bloodstream, macrophages settle in various tissues throughout the body.
- Do natural killer cells require stimulation from other cells to function?
No, natural killer cells possess natural cytotoxicity, allowing them to kill tumor cells and virus-infected cells without prior stimulation.
- Can NK cells differentiate between healthy cells and infected cells?
Yes, NK cells can identify cells with low levels of class I MHC molecules, which often indicate cellular abnormalities.
- Do phagocytes only eliminate bacteria and viruses?
Phagocytes, particularly macrophages, can engulf and eliminate various pathogens, including bacteria, viruses, and protozoans.
- How do NK cells contribute to the immune response?
NK cells secrete gamma interferons upon activation, which further activate other immune cells like macrophages and enhance the overall immune response.