Chapter 21

Immune System

Immune system• Immunology = study of the immune system• Immunity - elaborate defense system of the

human body• Immune system functions:

• protects the body from pathogens• protects the body from all other foreign

agents; pollens (ragweed),toxins (bee stings), own cells that have gone astray (cancer cells)

Classification of the Immune system

• Non-specific immunity - • Specific immunity-

Non-specific ImmunityBody does not need to recognize the

specific foreign agent• First Line of Defense -

• Mechanical barriers - intact skin, mucous membranes...pathogens can’t cross to enter body

• Chemical barriers - secretions of the mucous membranes assist in defensive function• Ex. tears, sweat, saliva wash away microorganisms, tears

establish hostile environment to preventing colonizing • Certain reflexes -

• sneezing, coughing help remove pathogens from the respiratory tract

• vomiting, diarrhea help remove pathogens from the digestive tract

Second Line of Defense

• Phagocytosis• Inflammation• Fever• Protective Proteins• Natural Killers

Phagocytosis• Phagocytose - eat, destroy pathogens

• Wandering phagocytes - neutrophils/monocytes are signaled by injured cells (chemotaxis)...neutrophils/monocytes travel through the blood to site of infection...then squeeze through gaps in the epithelial tissue (diapesis)

• Fixed phagocytes - monocytes deposit themselves in various organs (liver, spleen, lungs, lymph nodes)...give rise to macrophages, the “big eaters”

Inflammation• Bodies response to an irritant• Irritant: friction, heat, cold, radiations, chemicals, injuries

• when pathogens irritate the body...infection• Characterized by: redness, swelling, heat, pain• Caused by: Inflammatory response

• Injured cells release histamine, causes blood vessels to dilate, more blood flow to area (heat) (redness)

• Histamine also causes the leak fluid and dissolved substances into the tissue spaces (swelling)(pain)...

Inflammatory Response

Fever (pyrexia)

• Pyrogens - fever producing substances released by phagocytes as their perform their duty

• Pyrogens stimulate the hypothalamus to reset the body temperature...producing a fever

• Fevers are beneficial in two ways:• stimulates phagocytosis• decreases the ability of certain pathogens to

multiply

Fever• Elimination of the fever may do more

harm than good..• Studies show that the reduction of fever

prolongs and infection however... high body temperatures may cause severe, irreversible brain damage. High fevers in children usually are accompanied by a seizure (febrile seizure).

Protective Proteins• 2 groups of protective proteins:

• Interferons• group of proteins secreted by the cells that are infected by

the virus• they interfere with viral replication• activate the NK cells and macrophages ...boosting the

immune system

• Compliment proteins• circulate in the blood in an inactive form• when activated...they swarm over the bacterium, attach to

the bacterium's outer membrane, punch holes in the membrane allowing fluid and electrolytes to enter...burst...die

• activated compliments perform other functions that enhance phagocytosis and the inflammatory response

Natural Killers (NK)

Specific ImmunityThird Line of defense

• Specific immunity targets a foreign substance and provides protection against one specific substance... but no others

• 2 Cells that play a key role:• Lymphocytes (B lymphocytes, T lymphocytes)• Macrophages

• Understand Antigens to understand Lymphocytes

Antigen• Antigen is a substance that stimulates the

formation of antibodies• Antigens are found on the surface of many

substances...pathogens, red blood cells, pollens, food, toxins, and cancer cells

• Foreign substances that display antigens are described as antigenic

• Antigenic substances are attacked by lymphocytes!!

•The key to a healthy immune system is its remarkable ability to distinguish between the body’s own cells (self) and foreign cells (non-self). The body’s immune defenses normally coexist peacefully with cells that carry distinctive "self" marker molecules. But when immune defenders encounter cells or organisms carrying markers that say "foreign," they quickly launch an attack.

Lymphocytes• T Lymphocytes (T Cells) - 50% of lymphocytes travel to

the thymus gland to mature and differentiate...eventually they leave the thymus gland to lymphoid tissue (spleen, lymph nodes) and some circulate in the blood

• B Lymphocytes (B Cells) - differentiate in fetal liver and bone marrow...take up residence in lymphoid tissue

• Both T cells and B cells attack antigens ...in different ways

• T cells attack antigens directly (cell-to-cell contact)• B cells interact with the antigen indirectly...through the

secretion of antibodies called antibody-mediated immunity

Antibodies• Antibodies are proteins secreted by the B

Cells called Immunoglobulins• Immunoglobulins are found primarily in the

plasma• 3 most abundant (total 5)

• IgG (immunoglobulin G)• IgA (immunoglobulin A)• IgM (immunoglobulin M)

What do antibodies do?• Antibodies destroy pathogens in 2 ways...• Directly: bind to antigens (antibody-antigen reaction)

• Agglutination (clumping together) occurs making it easier for the phagocytic cells to destroy the cells

• Indirectly: activation of the compliment proteins• cause a variety of effects:

• chemotaxis (chemical attraction of more phagocytes)

• promotes agglutination...more phagocytosis• encourages lysis (rupture of the pathogen)

Primary and Secondary response

• Primary response - initial response to an antigen• when the B cells produce many plasma cells

and memory cells --remember plasma cells secrete antibodies

• associated with low levels of plasma antibodies• Secondary response - when exposed to the same

antigen for a second time...• immune system responds quickly and produces

a larger amount of antibodies

Secondary response benefits

• Secondary response means you are immune!

• Example... if you had measles as a child, you already developed antibodies and many memory cells...if exposed again, memory cells quickly call to action the antibody secreting plasma cells

• Level of antibodies in your blood is called an antibody titer

Types of Immunity• Genetic• Acquired

• Naturally acquired (active)• Passive

• Artificially acquired• Vaccine• Immune globulin • Antitoxins, Antivenoms

Allergic Reactions

Autoimmune Disease

Meet Your Immune System• Germs are awful things that get inside your bodies

through the nose, mouth, scrapes, and any other way they can get in.   Although the fevers mean your immune system is fighting against germs, you probably wish there were other ways to show that they are fighting.

• The first immune fighters at the site are the phagocytes.  Some release a chemical that causes a fever.  That heat is another way of killing some germs, because they prefer a temperature of 98.6 degrees.

• White blood cells are first on the spot along with the phagocytes and lymphocytes.  They help destroy the germs, but also they warn the rest of your body that germs have invaded.

• Helper T-cells are in the command.  They control the entire immune T-cell system and give orders about how to destroy the germ.

• Then the Killer T-cells arrive.  T-cells are made in the marrow of our bones.  They stay there until they are needed.  Then they go to the thymus to get circulated into the blood searching for germs.  When they find germs, the Killer T-cell helps the phagocytes kill the germ.

• When Helper T-cells call for the B-cells they come out of the bone marrow.  B-cells are made in the marrow of your bones.  They stay there until T-cells call for them. 

• Each germ has proteins called antigens on its surface.  Antibodies are proteins that attach to the germ's antigens.  Think of it like a boat dock.   If one boat docks, another cannot fill the same space.  Antibodies fill the germ's dock so it can not attach to a cell and infect it.  Antibodies are made by the B-cells. 

• Suppressor T-cells get there when the battle is over and it looks safe. They tell the Killer T-cells to stop fighting. Inhibitors!

• Memory T-cells are sent after the Suppressor T-cells.  They patrol the area and watch for the germ's return.   If the germ returns, they alert the system which already has a strategy for fighting this germ.