Lecture 19: Immune System

Covers Chapter 36

The world is teeming with microbes!!!

• When microbes cause disease, they are called pathogens.

• Four types that can invade humans:– Bacteria– Viruses– Protists– Fungi

Modes of Transmission

• Direct Contact– Shake hands with someone who is sick

• Indirect Contact– Touch a contaminated object: telephone, doorknob

• Vector-Borne Transmission– Intermediate organism gives it to you: mosquitos (malaria),

bat (rabies)• Inhalation

– Someone sneezes on you• Ingestion

– Usually food poisoning: salmonella (chicken), beef (E.Coli)

The Lymphatic System

• A large part of the Immune System is the Lymphatic System

• Purpose of the Lymphatic System*:– 1.) Removal of excess fluid from the tissues

(Conducting System)• Via lymph vessels, capillaries, ducts. Carries

fluid back to heart from the body.– 2.) Production of immune cells (Lymphoid Tissue)

• Made in Bone Marrow and Thymus Gland and housed in lymph nodes, spleen, nodes and glands

thymus

spleen

bone marrow

thoracic duct

valve preventsbackflow

lymph node

chambers packedwith white blood cells

lymph vessels

lymph nodes

The Lymphatic System Contains Much of the Immune System

Fig. 36-5

Lymph Node Close-up

Lymphatic System Components

• Tonsils: a specialized lymph node around the pharynx

• Thymus gland: small gland in chest below breastbone. Lymphatic cells mature here

• Bone Marrow: produces some lymphatic cells• Spleen: fist-sized organ on left side of body. It

filters blood, exposing it to lymphatic cells stores and purifies blood, high concentration of lymph cells

The Body’s Immune Responseto Pathogens*

• Three lines of defense:1.) Non-specific External Barriers (skin and

mucus membranes)

2.) Innate Immune Response (WBC’s, Proteins, Inflammation)

3.) Adaptive Immune Response (Lymphocytes: T cells and B cells)

Levels of Defense Against Disease

Fig. 36-1

If these barriers are penetrated,the body responds with

If the innate immune response is insufficient,the body responds with

Adaptive Immune Responsecell-mediated immunity, humoral immunity

Nonspecific External Barriersskin, mucous membranes

Innate Immune Responsephagocytic and natural killer cells,

inflammation, fever

1.) Non-specific external barriers(Skin and Mucus Membranes)*

• NON-SPECIFIC DEFENSE (does not distinguish one pathogen from another-fight all pathogens the same way)– Skin: low pH of skin & sweat from skin glands helps

repel pathogens– Saliva, tears, & mucus membranes in

respiratory/GI/urinary/reproductive tract secrete enzymes that:

• Slow growth of pathogen

• Break down pathogens

• Trap pathogens so they can be washed out of body

The Protective Function of Mucus

Fig. 36-2

Bacteria trappedby mucus and cilia

2.) Innate Immune Response(WBC’S, PROTEINS, INFLAMMATION)*

• Also a NON-SPECIFIC line of defense: If a pathogen gets through the first line of defense, then 2nd line goes to work: – TWO types of WBC’s– TWO types of proteins– The Inflammatory Response

WBC’s*

Macrophages: move through lymph fluid, kill pathogens through phagocytosis** (engulf & kill with enzymes then eject material.) Discarded material causes inflammation and swelling of lymph nodes when we are sick.

Natural Killer Cells: kill human cells invaded by pathogens via cell lysis** (poke holes cell membrane and pump in enzymes to kill the cell.)

The Attack of the Macrophages

Fig. 36-3

Proteins*

Interferons: secreted by cells infected by viruses. They act as a coating on nearby healthy cells to protect them from getting attacked.***

Complement Proteins: circulate in the blood. They coat the surface of a pathogen, making it “easier” for the macrophage to see.**

Inflammatory Response

• Damaged cells release chemicals that start a cascade, resulting in– Redness, swelling and pain

• Histamine relaxes smooth muscle in vessels, allowing more blood flow to area and causing capillary walls to become leaky: redness, swelling, pain

– Macrophages attracted to the area of damage

– Blood clotting (if skin broken)

The Inflammatory Response

3.) Adaptive Immune ResponseLymphocytes: T & B Cells

• This is a SPECIFIC line of defense*• T & B cells go after specific pathogens*• T&B cells start as stem cells in Bone Marrow• B cells remain in bone marrow until mature

then emerge • T cells leave bone marrow, travel to thymus

gland to mature. Then they emerge..• Mature T & B cells “hang out” in lymphatic

system: lymph nodes, glands and spleen.

How do T & B cells recognize pathogens*

• Pathogens have unique molecules on their surface, which human immune cells can recognize. These molecules are called antigens (proteins, polysaccharides or glycoproteins).

• B cells and T cells (and macrophages) have ANTIGEN RECEPTORS on their surface. They recognize antigens on surface of pathogens OR placed on the surface of an infected human cell (pathogens can “hijack” cells and “paste” an antigen on a human cell.) – The antigen receptors belonging to B cells are called antibodies.– The antigen receptors belonging to T cells are called T-cell receptors.– These receptors are y-shaped proteins whose “ends” serve as

antigen receptor sites

Antigen Receptors on T & B cells

Antibodies on B Cells

Fig. 36-7

(a) Antibody receptor function

B cell

antibody

antigen

mic

robe

T cell receptor

B-Cells: Antigen-Antibody Response

Pathogen enters body and B-cell encounters it

B cells kill mostly small pathogens (bacteria or fungi usually)*

FOUR THINGS HAPPEN: • 1.) B-cell will cause destruction of pathogen.*

B-cell antigen receptor (antibody) recognizes an antigen on the pathogen and this interaction signals complement to come in and coat the pathogen, making it a target for macrophage.

B-Cells: Antigen-Antibody Response

• 2.) B cell will be stimulated to replicate itself* at high numbers with the same antigen receptor (antibody) placed on the surface of the new B cells (called plasma cells)

B-Cells: Antigen-Antibody Response

• 3.) B cells secrete antibodies* (free-floating copies of the antigen receptor)

• 4.) In addition, memory B-cells are created* so that a faster immune response will happen if pathogen attacks body again

B Cell Antigen-Antibody Response

Fig. 36-10

Invading antigensbind to antibodies onone B cell (dark blue)

1

The B cell “selected”by the antigen multipliesrapidly

2

A large clone ofgenetically identicalB cells is produced

3

These B cellsdifferentiate intoplasma cells andmemory B cells

4

Plasma cellsrelease antibodiesinto the blood

5endoplasmic

reticulum

memory Bcell

plasma cell

antibodies

antigensantibodies

T Cells

• T Cells- attack HUMAN CELLS that have been infected by microorganisms AND LARGE microorganisms (viruses usually)*

T-Cells*• There are 3 different kinds:

1.) Cytotoxic T-cells• Virus enters one of our cells• VIRUS WILL PLACE ITS ANTIGEN ON SURFACE OF

CELL • Cytotoxic T-cells recognize antigen and attack cell

(punch holes & pump in enzymes)

2.) Helper T-Cells• Send chemical “message” to other Cytotoxic T-cells &

B-cells about the presence of the pathogen

3.) Memory T-Cells• These cells are created for faster immune response if

same pathogen attacks again

T Cells

Fig. 36-13, 1 of 2

B cell helper T cell cytotoxic T cell

cytokines

infected cell

dendritic cellor macrophage

antibody

viralantigen

Targets invaders outside cells (e.g.,viruses, bacteria, fungi, protists, andtoxins)

Stimulate both humoral and cell-mediatedimmunity by releasing cytokines

Targets defective body cells (e.g., infectedcells and cancer cells), transplants

HUMORAL IMMUNITY CELL-MEDIATED IMMUNITYHELPER T CELLS

B-cell antibodies bind to viral antigens and stimulate the B cells to divide and differentiate

Viral antigens presented on the surfaces of dendritic cells or macrophages, and infected cells

T-cell receptors bind to viral antigens

Cytokines released by helper T cells stimulate B cells and cytotoxic T cells

virus

T Cells

Fig. 36-13, 2 of 2

memory cytotoxic

T cell

memoryhelperT cell

memory B cell

infectedcell

cytotoxic T cellplasmacell

Plasma cells secrete antibodies into the blood and extracellular fluid

Memory cells confer future immunity to this virus

Cytotoxic T cells release pore-forming proteins that destroy infected cells

B cell helper T cell cytotoxic T cell

cytokinesantibody

Targets invaders outside cells (e.g.,viruses, bacteria, fungi, protists, andtoxins)

Stimulate both humoral and cell-mediatedimmunity by releasing cytokines

Targets defective body cells (e.g., infectedcells and cancer cells), transplants

HUMORAL IMMUNITY CELL-MEDIATED IMMUNITYHELPER T CELLS

Cytokines released by helper T cells stimulate B cells and cytotoxic T cells