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The Maintenance of Balance by the

Immune System

Also Known As…Why the aliens died at the end of War of the Worlds and we

didn’t.

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The Lines of Defence•Your body has three lines of defence that it

uses to defend you from invading pathogens.•A pathogen is the term used to describe

any disease-causing organism and/or entity.

•These three lines of defence are…–The External Defence–White Blood Cells–Antibody Formation

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External Defenses•The first line of defence is mainly physical

with some chemical warfare on the side.•Skin – Impenetrable layer surrounding the body.

The skin has a low pH of 3-5.•Mucus, Saliva & Sweat – These chemicals

surround open areas and contain lysozyme – an antimicrobial enzyme.

•Cilia – Tiny hair-like projections on cells that trap invaders and sweep them back out of the body.

•Stomach acid/enzymes – Some pathogens, that are ingested, make their way down the throat hit this area and soon die because of the harshness of the environment.

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Second Line of Defence (I)•The second line of defence is made of

leukocytes – white blood cells (WBC’s).•Phagocytosis may be used by WBC’s to

engulf and destroy pathogens. There are WBC’s called macrophages that do this.

•Neutrophils (kamikazes) respond to a chemical signal released by damaged cells and engulf invaders and release hydrolytic enzymes. The left-overs from this battle is pus.

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Macrophages in Action!

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Second Line of Defence (II)•The action of the WBC’s initiates the

inflammatory response – a non-specific response triggered by damaged cells that includes swelling, redness, heat and pain. All of these are associated with increased blood flow and delivery of WBC’s.•Fever – The hypothalamus turns up

the heat to about 40° which makes it hard for invaders to survive.

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Third Line of Defence•The third line of defence involves specific

identification and attack through complement proteins and antibody formation.

•Complement proteins float freely in the blood and are inactive until a pathogen is present. The complement recognizes marker proteins on the pathogen and do one of the following…

•Coat & immobilize pathogen.•Puncture pathogen so it swells and bursts.•Attach to pathogen and attract leukocyte for

phagocytosis.

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Antibody Formation

•Antibodies are proteins that target, inactivate and destroy antigens – the proteins on the surface of pathogens.

•Antibodies are made by specialized WBC’s called lymphocytes.

•There are two types of lymphocytes…•T Cells - made in the bone marrow and matured in

the thymus gland (T for thymus). The role of the T cell is to seek out intruders and signal the attack. Tattle-tales!

•B Cells - made and processed in the bone marrow. Each B cell makes a single type of antibody. Some B cells can become plasma cells that can produce as many as 2000 antibodies. Blasters!

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What Does the Antibody Do?

•The antibody is a Y-shaped molecule that has two arms that attach to the antigen – the two proteins match like key-n-lock.

•The antibody…•Enlarges the pathogen – making it more

conspicuous to WBC’s.•Changes pathogen shape to disallow entry into

cells. No getting through a protein portal unless you have the perfect shape.

•HIV infects T cells so they can’t be tattled on! They match up with and hide inside the T’s.

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Antibodies & Antigens

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How Do the T’s & B’s Work?

1. A macrophage will engulf a pathogen and push its antigens to the outside where the T cells can identify the antigen.

2. The T cell makes a “blueprint” of the antigen.

3. The T’s go to the B cells which read the “blueprint” and make the matching antibody. Many copies of this antibody are made.

4. Antibodies attach to antigen of invading pathogens…you know what happens from here.

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T’s and B’s in Action!

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Other Interesting Immune Cells

•Killer T cells - seek and destroy microbes and cells that are infected by viruses so the virus is destroyed and cannot reproduce.

•Suppressor T cells - signal the immune system to shut down and conserve energy once the battle has been won.

•Memory B cells - generated during the infection which holds an imprint of the antigen. The immune system is capable of recording the information about an invading antigen so it does not get a foothold on your body again.

•Most T cells and B cells die during the infection or die off within a few days but the memory B cells remain and quickly recognize any invader should they come back.

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The Overall Picture

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FIN