Imagine an enzyme so powerful it can dissolve bacteria in seconds. That's lysozyme — one of your body's most important chemical weapons.

The Discovery

Lysozyme was discovered by Alexander Fleming in 1921 — the same scientist who later discovered penicillin. Legend has it that he had a cold and added some of his own nasal mucus to a bacterial culture, then noticed the bacteria were destroyed. He had discovered lysozyme, a natural antibacterial enzyme!

Where Lysozyme Is Found

Lysozyme is present in all your body's secretions:

• Tears — protecting your eyes from infection

• Saliva — protecting your mouth

• Nasal mucus — protecting your respiratory tract

• Breast milk — protecting newborns

• Sweat — protecting your skin

How Lysozyme Works

Lysozyme attacks the peptidoglycan layer in bacterial cell walls. Peptidoglycan is a mesh-like structure that gives bacteria their shape and protects them from bursting due to osmotic pressure.

Lysozyme breaks the bonds holding peptidoglycan together, like cutting the ropes that hold a tent up. Without this support, the bacterial cell wall collapses, and water rushes in by osmosis, causing the bacterium to burst (lyse).

Which Bacteria Are Affected?

Lysozyme is most effective against Gram-positive bacteria, which have a thick peptidoglycan layer exposed on the outside. Gram-negative bacteria have an outer membrane that protects their peptidoglycan, making them more resistant to lysozyme.

Your stomach produces hydrochloric acid (HCl) so powerful it can dissolve metal! This creates a lethal chemical barrier that destroys most pathogens you swallow with food and water.

The Numbers

• pH 1.5-3.5 — extremely acidic (stomach acid is more acidic than lemon juice!)

• 1.5-2.5 liters produced daily

• Produced by parietal cells in the stomach lining

How Gastric Acid Kills Pathogens

1. Denatures proteins — Acid unravels the 3D structure of bacterial proteins, destroying their function

2. Disrupts membranes — The extreme acidity damages bacterial cell membranes

3. Activates pepsin — Converts inactive pepsinogen to pepsin, a protein-digesting enzyme that also attacks bacteria

Why Your Stomach Doesn't Digest Itself

Your stomach is protected by a thick mucus layer (secreted by goblet cells) and by rapid cell turnover. The stomach lining renews every 3-5 days! When this protection fails, you get ulcers.

Pathogens That Survive

Some pathogens have evolved to survive gastric acid:

• Helicobacter pylori — produces urease enzyme that neutralizes local acid (causes stomach ulcers)

• Salmonella — can survive acidic conditions

• Vibrio cholerae — causes cholera

• Listeria — causes food poisoning

This is why "risky" foods can still cause illness despite passing through your stomach's chemical barrier.

Defensins are small antimicrobial peptides — protein fragments that act like molecular hole-punchers, creating lethal leaks in microbial membranes.

Where They're Produced

• Epithelial cells — in skin, respiratory tract, gastrointestinal tract

• Neutrophils — stored in granules, released during infection

• Paneth cells — specialized cells in the intestinal crypts

Types of Defensins

Alpha-defensins: Found in neutrophils and Paneth cells. Stored in granules and released when needed.

Beta-defensins: Produced by epithelial cells throughout the body. Continuously secreted as part of the barrier defense.

How Defensins Work

1. Defensin molecules are attracted to microbial membranes (which are negatively charged)

2. They insert themselves into the membrane

3. Multiple defensins cluster together, forming pores (holes)

4. The membrane becomes leaky

5. Essential contents escape; the microbe dies

Think of defensins as molecular hole-punchers that turn bacterial membranes into Swiss cheese!

Broad-Spectrum Activity

Defensins are effective against:

• Bacteria — both Gram-positive and Gram-negative

• Fungi — including Candida species

• Viruses — some enveloped viruses are susceptible

Because they attack the membrane (a fundamental structure), microbes have difficulty developing resistance to defensins.

The complement system is a collection of over 30 proteins that circulate in your blood in an inactive form. When activated, they work together like dominos falling — each protein activates the next in a cascade.

Three Activation Pathways

Classical pathway: Triggered by antibodies bound to pathogens (links to adaptive immunity)

Alternative pathway: Triggered directly by pathogen surfaces (innate immunity)

Lectin pathway: Triggered by mannose-binding lectin recognizing certain sugars on pathogens

All three pathways converge on the same endpoint — activation of C3, the central complement protein.

Three Main Functions

1. Opsonization — Making Pathogens "Tasty"

Complement fragment C3b coats the surface of pathogens. Phagocytes have receptors for C3b, making them much more likely to engulf the coated pathogen. It's like putting a "eat me" sign on bacteria.

2. Inflammation — Recruiting Reinforcements

Fragments C3a and C5a act as chemical signals that attract neutrophils and monocytes to the infection site. They also cause mast cells to release histamine, promoting inflammation.

3. Cytolysis — Direct Killing

The terminal complement components (C5b, C6, C7, C8, C9) assemble into the Membrane Attack Complex (MAC) — a ring-shaped structure that punches holes in pathogen membranes. Water enters, and the pathogen bursts.

Why Doesn't Complement Attack Our Own Cells?

Your cells have protective proteins on their surface that break down any complement that tries to attach. Only cells lacking these protective proteins (like bacteria) are vulnerable.