Page 162 - 84 human physiolofy part-1

Chapter 9

9 THE IMMUNE SYSTEM

live version • discussion • edit lesson • comment • report an error

Overview

T he immune system is a complex system that is responsible for protecting us against infections and

foreign substances. There are three lines of defense: the first is to keep invaders out (through skin,
mucus membranes, etc), the second line of defense consists of non-specific ways to defend against
pathogens that have broken through the first line of defense (such as with inflammatory response and
fever). The third line of defense is mounted against specific pathogens that are causing disease (B cells
produce antibodies against bacteria or viruses in the extracellular fluid, while T cells kill cells that have
become infected). The immune system is closely tied to the lymphatic system, with B and T
lymphocytes being found primarily within lymph nodes. Tonsils and the thymus gland are also
considered lymph organs and are involved in immunity. We often don't realize how effective the
immune system is until it fails or malfunctions, such as when the lymphocytes are attacked by HIV in
an AIDS patient.

The Immune System as a Castle

The immune system is a silent wonder. While we are very aware of our heart beating and the
breaths we take, we are much less aware of our immune system that protects us from thousands of
potentially deadly attacks every day.

In this chapter we will discuss the immune system we each possess that is working around the
clock, protecting us from disease and death.

A good way to start understanding the immune system is to liken it to a castle. A castle, like our
bodies, is a fortress. A castle has three lines of defense:

• First, A moat and drawbridge. The first line of defense in our bodies are physical and
chemical barriers - our skin, stomach acids, mucous, tears, vaginal opening, of which the last
three mostly produce lysozyme to destroy harmful incoming pathogens.
• Second, Sentries and archers who stand on the castle wall. In our bodies the second line of
defense is non-specific immune responses - marcrophages, neutrophils, interferons, and
complement proteins. This line of defense also includes fever and inflammatory response as
nonspecific defenses.
• Third, Soldiers within the castle.Our third line of defense is specific immune responses - T
Cells and B Cells. There are many types of each which work like a close knit team to destroy
pathogens.

If pathogens (invaders) try and succeed in penetrating the first line of defense, then the second line
of defense is ready to act. If both the first and second line of defense fail, then the third line of defense
will act. It is when all three lines of defense are breached that we get sick and are subject to disease.

So what we are trying to say is that the immune system is a set of mechanisms of defense,

162 | Human Physiology