Lysozyme, also known as muramidase, is a small enzyme that functions as a powerful, non-specific defense mechanism within the human body. This protein is a fundamental part of the innate immune system, acting as a natural antibiotic against bacteria. Its primary function involves a process called hydrolysis, where it chemically breaks down the peptidoglycan layer that forms the cell wall of many bacteria, causing the cell to rupture and die. Lysozyme’s widespread presence across various human fluids and cells demonstrates the body’s comprehensive strategy for neutralizing potential pathogens.
Lysozyme in Surface Secretions
The human body deploys lysozyme as a first line of defense in numerous surface secretions, creating an immediate barrier against environmental microbes. Tears, for instance, contain a significant concentration of lysozyme, providing continuous ocular defense against airborne bacteria attempting to enter the eye. This enzyme helps maintain the sterility of the eye’s surface, preventing infections.
Saliva, another prominent surface secretion, utilizes lysozyme to protect the oral cavity and upper digestive tract. The enzyme works alongside other antimicrobial agents in saliva to control the bacterial load, contributing to overall oral hygiene and reducing the risk of infection. Similarly, the mucus lining the nasal passages and respiratory tract is rich in lysozyme, acting to trap and chemically neutralize inhaled bacteria before they can establish an infection in the lungs.
Lysozyme within Immune Cells
Lysozyme also plays an internal, destructive role within the specialized cells of the immune system known as professional phagocytes. Key cells, including neutrophils and macrophages, store and utilize large amounts of this enzyme for intracellular bacterial destruction. Neutrophils, a type of white blood cell, contain lysozyme concentrated within their cytoplasmic granules.
When these immune cells engulf a bacterium through a process called phagocytosis, they trap the microbe inside an internal compartment called a phagosome. Lysozyme is then released from the cell’s granules directly into this compartment, where it works with other toxic molecules to dismantle the ingested bacterium. Macrophages, another type of phagocytic cell, also utilize lysozyme for this purpose, but they also constitutively secrete it outside the cell to aid in the clearance of local pathogens at infection sites.
Lysozyme in Specialized Fluids
Beyond surface secretions and immune cells, lysozyme is found in other specialized fluids where it serves distinct protective functions. Human breast milk contains a comparatively high concentration of lysozyme, often ranging from 200 to 500 mg/L in mature milk, which is significantly higher than in the milk of many other mammals. This enzyme provides passive immunity to the nursing infant, helping to protect the developing gastrointestinal tract from harmful bacteria and supporting the establishment of a healthy gut microbiome.
Lysozyme is also present in blood serum and plasma, indicating a systemic function throughout the body’s circulatory system. While the concentration in serum is much lower than in surface secretions, its presence suggests a role in maintaining the integrity of the blood and in clearing bacterial debris. Furthermore, its levels in the blood can be used as a marker for certain physiological states, such as inflammation or specific types of cancer.