39. Immune System & Defences

The human body has a multi-layered defense system to protect against pathogens. This system is divided into physical and chemical barriers that prevent entry, and the immune system which destroys pathogens that manage to get inside.

1. Physical and Chemical Barriers

These are the body's first line of defense:

• Skin: Acts as a physical wall. It also secretes antimicrobial oils that kill pathogens.

• Nose: Contains hairs and mucus to trap particles and pathogens from the air.

• Trachea and Bronchi: Lined with mucus to trap pathogens. They also have cilia (tiny hair-like structures) that move the mucus up to the throat to be swallowed.

• Stomach: Produces hydrochloric acid (pH 2), which is strong enough to kill most pathogens that enter through food or swallowed mucus.

2. The Immune System (White Blood Cells)

If pathogens breach the initial barriers, white blood cells take over. They have three primary methods of defense:

Phagocytosis

• Certain white blood cells (phagocytes) track down, bind to, and engulf (consume) pathogens to destroy them.

Producing Antitoxins

• Pathogens often produce toxins (small poisons) that damage cells.

• White blood cells produce antitoxins that bind to and counteract these toxins, rendering them harmless.

Producing Antibodies

• Antigens: Unique substances on the surface of a pathogen (like a protein or cell wall) that the immune system identifies as "foreign."

• Antibodies: Proteins produced by white blood cells that lock onto specific antigens.

• Action: Once locked on, antibodies act as signals for other white blood cells to destroy the pathogen.

• Specificity & Memory: Each antibody is specific to one type of antigen. Once the body identifies the correct antibody, it can produce it rapidly. The system "remembers" the pathogen, providing immunity if the same pathogen attacks again in the future.

Summary Table: Defense Mechanisms