The Immune System

What is its function?

PROTECTION FROM INVADERS! Three Lines of Defense:

Innate Immunity• 1. External Defenses - BROAD• 2. Internal Defenses - BROAD

3. Acquired Immunity - SPECIFIC

1. External Defenses

The Skin and Mucous MembranesPhysical barriers; trap microbesSecrete substances (oil, sweat, etc.)

that makes the skin too acidic for microbes to live there

Also secrete lysozyme, an enzyme that can destroy bacterial cell walls

This first line of defense occurs INNATELY

2. Internal Defenses

If an invader gets inside the body, the internal defenses (2nd line of defense!) take overA) Phagocytes

• White blood cells that “eat” invaders• Invaders (bacteria/virus/etc) are then

digested by lysosome

2. Internal Defenses

B) Interferons• Special proteins that virus-infected body cells

send out as a “warning” to other body cells• The other body cells then make other

substances to protect themselves from the viruses

2. Internal Defenses

C) Inflammatory Response Damage to tissue (from physical injury or the

entry of pathogens) leads to inflammation• Histamine is released• This causes INCREASED BLOOD FLOW to the area

• This brings clotting factors, platelets, phagocytes, etc. to help REPAIR

• (ie. Highway during evacuation)

2. Internal Defenses

D) Natural Killer Cells• Patrol the body and attack virus-infected

cells and cancer cells• Surface receptors (“nametags”) identify these

cells

All 4 of these internal defenses (phagocytes, interferons, inflammatory response, and natural killer cells) occur INNATELY

3. Acquired Immunity

Acquired immunity is the third line of defense

Acquired immunity only comes after EXPOSURE to a specific pathogen

Acquired immunity occurs more slowly than innate immunityThink of yourself… making actual

words (learning!) took much more time than making sound (innate!)

3. Acquired Immunity

Acquired immunity is performed by lymphocytes Made in the bone marrow B-cells:

• mature in the bone marrow

T-cells: • mature in the thymus

3. Acquired Immunity

An antigen is a foreign molecule that is recognized by lymphocytes and causes them to respond An antigen is usually a

surface marker (“nametag”)

Lymphocytes Have antigen receptors

(100,000 on each cell!) that recognize a SPECIFIC antigen

SPECIFIC!!

3. Acquired Immunity B-lymphocytes are

responsible for the humoral immune response They are responsible for

pathogens OUTSIDE of cells (in body fluids, etc.)

Once a B-lymphocyte is “activated” by coming in direct contact with THEIR SPECIFIC ANTIGEN, they reproduce a lot (using clonal selection) in order to destroy the invader

3. Acquired Immunity These B-lymphocytes have

different jobs: Some are effector cells

• Make antibodies!• Antibodies are special proteins

that bind onto the ANTIGENS of the “invaders,” which flags them for destruction (usually by macrophages)

Others are memory cells• These cells live a long time, and

can respond quickly if this same antigen is seen again

3. Acquired Immunity

T-lymphocytes are responsible for the cell-mediated immune response They guard against invaders inside cells (viruses,

other pathogens) Cytotoxic T cells

• They are the effectors (“hit men”) of the cell-mediated immune response

• They kill infected body cells or other cells that don’t belong (like tumors)

• Some of these cells will become memory cells, so they can be reactivated if the pathogen “strikes again”

The Immune System

Key Features of the Immune SystemSpecificity

• Recognizes SPECIFIC invaders – species of bacteria, for example

• Due to ANTIGENS on the invader• “nametags”

Diversity• The immune system can respond to millions of

different invaders because it has so many different lymphocytes “on reserve”

The Immune System

Key Features of the Immune System Memory

• The immune system can “remember” antigens it’s seen before and react more quickly the second, third, etc. time it sees them

• Acquired immunity• Because of memory cells!

Self/Nonself Recognition• The immune system can distinguish between the body’s

own molecules from foreign molecules• Autoimmune disorders (example: AIDS) means that this

part of the immune system is not working – the immune system destroys the body’s own tissues

• HIV destroys cytotoxic T cells, so they can’t STOP the invasion!

Passive vs. Active Immunity

Passive Immunity Transferring antibodies from one person to

another, without the B-lymphocytes having to make them!

The person will already have the memory cells and antibodies, so the next response will be quicker!

Example: • Pregnant mother passes antibodies to her fetus

through the placenta

Passive vs. Active Immunity

Active Immunity Immunity to a specific pathogen

that comes because the immune system has seen it before!

Can come naturally• Been sick with the pathogen

before• Example: had measles before,

2nd time won’t take as long to respond

Can come artificially• Immunization• Edward Jenner - smallpox• Stimulates B-lymphocytes to

make antibodies AND memory cells

Allergies

Allergies are hypersensitivies of the immune system to an environmental molecule because of its ANTIGENS

Anaphylactic shockLife-threatening allergic reactions to

ingested or injected allergensPeanuts, shellfish, bee stings, etc.