CHAPTER 24The Immune System

Pathogens

• Disease causing agents such as bacteria, viruses, fungi, protozoans, and other parasites. ( NOT all microorganisms are pathogens)

Transmission of disease• Pathogens can be

transmitted to a host from reservoirs in four main ways: by direct contact, by an object, through the air, or by an intermediate organism called a vector.

Immune System

• Bodies primary defense against pathogens• Three cooperative lines of defense have evolved to

counter these threats.– Two of these are nonspecific - that is, they do not

distinguish one infectious agent from another.

1st line: Physical Barriers• non-specific defense• external barriers– skin & mucus membranes– excretions

• sweat • stomach acid • tears• mucus • saliva

– “lick your wounds”

Lining of trachea: ciliated cells & mucus secreting cells

2nd: Generalist, broad range patrols • Patrolling white blood cells– attack invaders that get through the skin• recognize invader by reading antigen

– surface name tag

– phagocyte cells• macrophages• “big eaters”

Macrophage “eating” bacteria

Lymph system Production of white blood cells & traps “foreign” invaders

2nd “circulatory” system lymph node

lymph vessels(intertwined amongst blood vessels)

The lymphatic system becomes a crucial battleground during infection

• a network of lymphatic vessels and organs– It returns tissue fluid to the

circulatory system– It fights infections

• Lymph nodes are key sites for fighting infection– They are packed with

lymphocytes and macrophages

Phagocytes

• Macrophages wander in the interstitial fluid– They “eat” any bacteria

and virus-infected cells they encounter

Phagocytes

yeastmacrophage

macrophage

bacteriawhite blood cells that eat

Antimicrobial Proteins

• Interferons: acts in warning other cells of a viral invasion

• Complement: help attract phagocytes to foreign cells and help destroy foreign cells

Natural Killer cells

• A type of white blood cell that destroy the body's own infected cells, may attack cells that form tumors

The inflammatory response mobilizes nonspecific defense forces

• Tissue damage triggers the inflammatory response (redness, swelling, warmth, pus) injured cells release chemical signals

• histamines– increases blood flow– brings more white blood cells to fight bacteria– brings more red blood cells & clotting factors to repair

• The inflammatory response can– disinfect tissues– limit further infection

Fever

• When a local response is not enough– full body response to infection– raises body temperature – higher temperature helps in defense• slows growth of germs• helps macrophages• speeds up repair of tissues

SPECIFIC IMMUNITY~ 3rd line of defense

• IMMUNE RESPONSE a recognition system that distinguishes “self”

from “non-self”• responds to foreign molecules called antigens• Stimulates the body to make antibodies: specific defensive proteins which help to

counter antigens in various ways

How are invaders recognized?• Antigens– chemical name tags on the surface of

every cell• “self” vs. “invader”

disease-causingbacteria

disease-causingvirus

one of yourown cells

antigens say:“I belong here”

antigens say:“I am an invader”

antigens say:“I am an invader”

Immunity=protection afforded to an organism by previous exposure to an antigen

• 2 types:1. Active: long term, conferred

by actual encounter with infection (naturally acquired) or vaccination *(artificially acquired)

* vaccine~ substance consisting of weakened, dead, ,or parts of pathogen or antigen that when injected , stimulates the immune system

Immunity

2. Passive: short term, antibodies passed on, not stimulated by antigens,

acquired naturally by placenta or breast milk or artificially acquired by a shot

Immune Response ~ 3rd line of defense

• Governed by 2 different parts of the immune system

• 1. Humoral Immunity: results in the production of antibodies

• 2. Cell-mediated Immunity: defensive activities are carried out by specialized cells circulating around the body

Lymphocytes ~type of white blood cell that carry out the immune response

• Two kinds– B cells secrete antibodies

that attack antigens (humoral immunity)

– T cells attack cells infected with pathogens (cell-mediated immunity)

B lymphocyte

The initial immune response results in a type of “memory”

• primary immune response produces memory cells– These cells may confer lifelong immunityWhen memory cells are activated by subsequent exposure to an antigen,

they mount a more rapid and massive secondary immune response

Humoral Immune Response

• Triggered by a specific antigen, a B cell differentiates into a plasma cell

-The plasma cell secretes antibodies

Antibodies are the weapons of humoral immunity

• An antibody molecule has antigen-binding sites specific to the antigen

• Do not posses the power to destroy antigens directly, tag and mark them for destruction by a variety of mechanisms

B cells & antibodies• B cells– white blood cells that attack

invaders in blood– mature in Bone marrow

• Patrolling B cells – make antibodies against invader immediately

• Memory B cells– remembers invader– can make antibodies quickly the next time• protects you from getting disease more than once

B cells immune responseinvader(foreign antigen)

Y

Y

Y

Y

B cellsY

Y

Y

Y

Y

YY

Y

Y

Y

Y

Y

Y

Y

Y

YY

Y

Y

Y

Y

Y

Y

YY

Y

Y

Y

“reserves”

memory B cells

Y

Y

Y

Y

Y

YY

YY

YY

Y

Y

B cellsrelease antibodiespatrol bloodforever

recognition

10 to 17 days

YY

Y

Y

YY Y

YY

Y

Y

Y

Y

YY

YYY

YYY

Y

Y

2006-2007

What if the attacker gets past the B cells in the blood &

infects some of your cells?You need trained assassins to kill

off these infected cells!

T

Attackof the Killer T cells!

T cells• T cells mature in Thymus• Helper T cells – sound the alarm for rest

of immune system

• Cytoxic or Killer T cells– destroy infected body cells

• Memory T cells– remembers invader & reacts against

it again quicklyWhere’s that?

Cell-Mediated Immunity• An antigen-presenting cell (APC) first displays a

foreign antigen and one of the body’s own self proteins to a helper T cell

• The helper T cell’s receptors recognize the self-nonself complexes on the APC

- The interaction activates the helper T cells– The helper T cell can then activate cytotoxic T cells with

the same receptors– Helper T cells also stimulate the humoral responses

• Cytotoxic T cells( or Killer T cells) bind to infected body cells and destroy them

Attack of the Killer T cells• Killer T cells destroy infected body cells– T cell binds to invaded cell– secretes perforating protein• punctures cell membrane of infected cell• cell bursts

Perforin puncturescell membrane

cell membrane

Killer T cell

cell membrane

invaded cell

vesicle

Cytotoxic T cells (Killer T) may help prevent cancer

– The surface molecules of cancer cells are altered by the disease

Immune responseinvader

invaders in blood invaders infect cells

B cells T cells

macrophages

helperT cells

patrollingB cells

memoryB cells

memoryT cells

killerT cells

YYY

Y

YY

Y

YantibodiesY

Y Y

skinskininvaders in body

YY

Y

Y

YY

Y

YantibodiesY

Y Y

The immune system depends on our molecular fingerprints

– “Self” is signaled by major histocompatibility complex (MHC), a group of proteins unique to the individual that is present on the surface of cells

• Can cause rejection of transplanted organs

DISORDERS OF THE IMMUNE SYSTEM

• Autoimmune diseases– The system turns against

the body’s own molecules

– Examples: Rheumatic fever, Type I Diabetes, Multiple Sclerosis, Rheumatoid arthritis, Lupus

Rheumatoid arthritisRheumatoid arthritis

DISORDERS OF THE IMMUNE SYSTEM

• Immunodeficiency diseases– Immune components

are lacking, and infections recur

Ex: Acquired immune deficiency syndrome (AIDS)

SCIDS

HIV on a lymphocyte

AIDS

– HIV (Human Immunodeficiency Virus) the virus that causes AIDS

– HIV is transmitted mainly in blood and semen

– HIV kills helper T-cells and leads to AIDS

DISORDERS OF THE IMMUNE SYSTEM

• Allergies are overreactions to certain environmental antigens called

allergens : Ex. Animal dander, dust mites, pollen

release of histamine causes symptoms

Antihistamines can relieve symptoms

anaphylactic shock: life threatening reaction to injected or ingested allergens..