Innate defenses – lysozyme distribution Lysozyme, the peptidoglycan-cleaving enzyme of innate immunity, is characteristically abundant in which of the following human body fluids?

Introduction / Context:
Lysozyme is an antimicrobial enzyme that cleaves the glycan backbone of bacterial peptidoglycan, compromising bacterial cell walls. Recognizing where lysozyme is abundant helps understand first-line barriers to infection at mucosal and glandular surfaces.

Given Data / Assumptions:

• Question asks for a characteristic site with high lysozyme levels.
• Body fluids listed include saliva, sweat, cerebrospinal fluid (CSF), and urine.
• Single best answer format.

Concept / Approach:

Lysozyme is classically abundant in secretions that bathe exposed epithelia: tears, saliva, nasal secretions, and to a lesser extent sweat and milk. Saliva is a prototypical example used in textbooks to illustrate lysozyme-mediated protection in the oral cavity. CSF can contain lysozyme but it is not emphasized as a primary antimicrobial secretion, and urinary lysozyme levels vary and are typically low under normal physiology.

Step-by-Step Solution:

Verification / Alternative check:

Clinical saliva protein profiles consistently show lysozyme alongside lactoferrin and secretory IgA; antibacterial activity against Gram-positive organisms is well documented for saliva-derived lysozyme.

Why Other Options Are Wrong:

• Sweat: contains lysozyme but at lower, variable levels; not as classic as saliva.
• CSF: may contain lysozyme, especially with inflammation, but is not a frontline external secretion.
• Urine: typically contains minimal lysozyme in healthy individuals; levels may rise pathologically.
• Bile: not a principal source of lysozyme.

Common Pitfalls:

• Assuming “present anywhere” equals “characteristically abundant.” Exams usually expect tears or saliva as the canonical answers.

Final Answer:

Saliva