The Human Immune System

Learning Objectives

Distinguish between infectious and non-infectious diseases.

Describe the body's three lines of defense against pathogens.

Explain the difference between the innate (non-specific) and adaptive (specific) immune response.

Understand the basic principle of how vaccines work.

The Body's Defense Force

Your body is constantly under attack from microscopic invaders like bacteria, viruses, and fungi. These disease-causing organisms are called pathogens. A disease that is caused by a pathogen and can be spread from one person to another is called an infectious disease. (Non-infectious diseases, like heart disease or diabetes, are not caused by pathogens and cannot be spread).

To protect you from these invaders, your body has a complex and powerful defense network called the immune system. It has three main lines of defense.

First Line of Defense: The Barriers

This is the innate (inborn) and non-specific defense system. Its goal is to keep pathogens from getting inside your body in the first place.

Skin: Your skin is a tough, waterproof physical barrier that most germs can't get through.

Mucous Membranes: The linings of your respiratory and digestive tracts produce a sticky fluid called mucus that traps pathogens. Tiny hairs called cilia in your airways sweep this mucus out.

Stomach Acid: The powerful acid in your stomach kills most of the pathogens that you swallow with your food and water.

Second Line of Defense: The Inflammatory Response

If a pathogen gets past the first line of defense (for example, through a cut in your skin), the second line of defense kicks in. This is also part of the innate, non-specific immune system.

Inflammation: The injured area becomes red, swollen, warm, and painful. This is a sign that your body is fighting back. Blood flow to the area increases, bringing in special white blood cells.

Phagocytes: These are a type of white blood cell that act as 'pac-men'. They engulf and digest the invading pathogens. A macrophage is a major type of phagocyte.

Fever: The body may raise its overall temperature. A higher temperature can slow down the growth of some bacteria and viruses and help the immune cells work better.

Third Line of Defense: The Immune Response

This is the adaptive or specific immune system. This system is incredibly smart. It can recognize a specific pathogen, attack it, and then remember it, so you won't get sick from the same germ again.

Antigens: Pathogens have unique molecules on their surfaces called antigens. Your immune system recognizes antigens as 'foreign'.

Lymphocytes (B-cells and T-cells): These are the special white blood cells of the adaptive immune system.

B-cells produce proteins called antibodies. Antibodies are like guided missiles. They find a specific antigen, lock onto it, and mark the pathogen for destruction.

T-cells can directly attack and kill infected body cells or help to activate the B-cells.

Memory: After the infection is defeated, some B-cells and T-cells become memory cells. They remain in your body for years, ready to launch a fast and powerful attack if that specific pathogen ever returns.

How Vaccines Work

A vaccine takes advantage of the third line of defense and its ability to remember. A vaccine contains a dead or weakened version of a pathogen, or just a piece of one. This is enough for your immune system to recognize the antigens and produce antibodies and memory cells against it, but it's not enough to make you sick.

If you are later exposed to the real, active pathogen, your memory cells will immediately recognize it and destroy it before it can cause disease. Vaccines provide immunity without you having to get the illness first.

Key Terms

**Pathogen: A bacterium, virus, or other microorganism that can cause disease.
**Infectious Disease: A disease caused by a pathogen that can be transmitted from one individual to another.
**Innate Immunity: The body's non-specific, general defense against pathogens (the first and second lines of defense).
**Adaptive Immunity: The body's specific defense system that recognizes, attacks, and remembers specific pathogens (the third line of defense).
**Antigen: Any substance that causes the immune system to produce antibodies against it.
**Antibody: A Y-shaped protein produced by B-cells that is used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses.
**Vaccine: A substance used to stimulate the production of antibodies and provide immunity against one or several diseases, prepared from the causative agent of a disease, its products, or a synthetic substitute, treated to act as an antigen without inducing the disease.