1. Chapter 43
THE IMMUNE SYSTEM

2. What you must know: Several elements of an innate immune response
The differences between B and T cells relative to their activation and actions.
How antigens are recognized by immune system cells
The differences in humoral and cell-mediated immunity
Why Helper T cells are central to immune responses

3. Types of Immunity Innate Immunity Adaptive Immunity Non-specific
All plants & animals
Pathogen-specific
Only in vertebrates
Involves B and T cells
Non-specific immunity – it is the nonspecific prevention of the entrance of invaders into the body.

4. Plant Defenses Nonspecific responses
Receptors recognize pathogen molecules and trigger defense responses
Thicken cell wall, produce antimicrobial compounds, cell death
Localize effects

5. Pathogens (such as bacteria, fungi, and viruses)
Figure 43.2
Pathogens (such as bacteria, fungi, and viruses)
INNATE IMMUNITY (all animals)
Barrier defenses:
Skin Mucous membranes Secretions
Recognition of traits shared by broad ranges of pathogens, using a small set of receptors •
Internal defenses:
Phagocytic cells Natural killer cells Antimicrobial proteins Inflammatory response
• Rapid response
Figure 43.2 Overview of animal immunity.
ADAPTIVE IMMUNITY (vertebrates only)
Humoral response:
Antibodies defend against infection in body fluids.
Recognition of traits specific to particular pathogens, using a vast array of receptors •
Cell-mediated response:
Cytotoxic cells defend against infection in body cells.
• Slower response

6. Innate Immunity Antimicrobial Proteins: Barrier Defenses:
Interferons (inhibit viral reproduction)
Complement system (~30 proteins, membrane attack complex)
Barrier Defenses:
Skin
Mucous membranes
Lysozyme (tears, saliva, mucus)
Innate Immunity
(non-specific)
Natural Killer Cells:
Virus-infected and cancer cells
Inflammatory Response:
Mast cells release histamine
Blood vessels dilate, increase permeability (redness, swelling)
Deliver clotting agents, phagocytic cells
Fever
Barrier Defenses:
Saliva contains an enzyme called lysozyme that can kill germs before they have a chance to take hold. Lysozyme is also present in our tears, providing a nonspecific defense mechanism for our physical barrier to infection.
The skin covering the entire body is a nonspecific defense mechanism – it acts as a physical barrier to infection. The mucous lining of our trachea and lungs prevent bacteria from entering cells and actually assists in the expulsion of bacteria by ushering the bacteria up and out with a cough. Finally, remember how we told you that you did not want to get out of the bus in the stomach? That is also the case for bacteria – it is a dangerous place for them as well. The acidity of the stomach can wipe out a lot of potential invaders.
Phagocytic WBCs:
Macrophages and neutrophils, roam the body in search of bacteria and dead cells to engulf and clear away.
Monocytes are another type of phagocytic leukocyte. They migrate into tissues and develop into macrophages.
Eosinophils: are leukocytes that defend against parasitic invaders such as worms by positioning themselves near the parasite’s wall and discharging hydrolytic enzymes.
Neutrophils: are WBCs that ingest and destroy microbes in a process called phagocytosis.
Dendritic cells populate tissues in contact with the environment, where they capture pathogens, display foreign antigens, and start primary immune response.
Antimicrobial Proteins:
Some assistance is offered to their cause by a protein molecule called complement. This protein makes sure that molecules to be cleared have some sort of identification displaying the need for phagocyte assistance. Complement coats these cells, stimulating phagocytes to ingest them. Cells involved in mechanisms that need cleanup assistance, such as platelets, have the ability to secrete chemicals that attract macrophages and neutrophils to places such as infection sites to help in the elimination of the foreign bacteria. They are nonspecific because they are not seeking out particular garbage… they are just looking for something to eat.
Interferon proteins provide innate defense against viral infections. They cause cells adjacent to infected cells to produce substances to inhibit viral replication.
The complement system consists of roughly 30 proteins with a variety of functions that enhance the immune response. One function is to lyse invading cells.
Inflammatory Response:
Tiny splinter – within our tissues lie cells known as mast cells. These cells contain the signal histamine that calls in the cavalry and initiates the inflammation response.
A local inflammatory response
is triggered by damage to tissue by physical injury or the entry of pathogens. It leads to release of numerous chemical signals. For example histamines are released by mast cells in response to injury. Histamines trigger the dilation and permeability of nearby capillaries. This aids in delivering clotting agents and phagocytic cells to the injured area. Systematic inflammatory responses include fever and increased production of white blood cells to fight infection.
Natural Killer Cells:
Help to recognize and remove diseased cells
Phagocytic WBCs:
Neutrophils (engulf)
Macrophage (“big eaters”)
Eosinophils (parasites)
Dendritic cells (adaptive response)

7. Phagocytosis

8. Inflammatory Response

9. Lymphatic System: involved in adaptive immunity

10. Adaptive Response
Lymphocytes (WBCs): produced by stem cells in bone marrow
T cells: mature in thymus
helper T, cytotoxic T
B cells: stay and mature in bone marrow
plasma cells  antibodies
They circulate through the blood and lymph, and both recognize particular antigens. All blood cells proliferate from stem cells in the bone marrow.

11. Antigen: substance that elicits lymphocyte response
Antibody (immunoglobulin – Ig): protein made by B cell that binds to antigens
Antigens:
B cells and T cells recognize them by specific receptors embedded in their plasma membranes.
B or T cell activation occurs when an antigen binds to a B or T cell. B cell activation is enhanced by cytokines. The lymphocytes forms two clones of cells in a process called clonal selection.
Two types of clones are produced: short-lived activated effector cells that act immediately against the antigen and long-lived memory cells that can give rise to effector cells if the same antigen is encountered again

12. Major Histocompatibility Complex (MHC)
Proteins displayed on cell surface
Responsible for tissue/organ rejection (“self” vs. “non-self”)
B and T cells bind to MHC molecule in adaptive response
Class I: all body cells (except RBCs)
Class II: displayed by immune cells; “non-self”
B cell receptors bind to intact antigens.
T cell receptors bind antigens that are displayed by antigen-presenting cells (APCs) on their MHCs.
MHC molecules are proteins that are the product of a group of genes. (Individuals differ in their MHC. This is a major component of “self”.) Responsible for stimulating the rejection of tissue grafts and organ transplants. The chance of successful transplantation are increased if the donor’s tissue-bearing MHC molecules closely match the recipient's. The recipient make also take immunosuppressant drugs.
The specificity of B and T cells is a result of the shuffling and recombination of several gene segments and results in more than 1 million different B cells and 10 million different T cells.
Each B or T cell responds to only one antigen.

13. Cell-Mediated Immune Response Humoral Immune Response
Antigen-presenting cell
Cell-Mediated Immune Response
(T Cells)
Humoral Immune Response
(antibodies)
Helper T cell
B cell
Cytotoxic T cell
Plasma cell
Humoral Immune Response (Antibodies):
Involves the activation and clonal selection of effector B cells, which produce antibodies that circulate in the blood.
Cell-Mediated Immune Response (T cells):
Involves the activation and clonal selection of cytotoxic T cells, which identify and destroy infected cells.
Two types of T Cells:
Helper T cells: aid both responses. When activated by interaction with the class II MHC molecule of an APC, they secrete cytokines that stimulate and activate both B cells and cytotoxic T cells.
Cytotoxic T Cells:
Bind to class I MHC molecules, displaying antigenic fragments on the surface of infected body cells. Cytotoxic T cells destroy infected body cells.
tag for destruction
Identify and destroy
Infected cell
Antibodies

14. Summary of Types of Cells
B cells: make antibodies, which provide humoral immunity. This helps fight pathogens that are circulating in body fluids.
Cytotoxic T cells: destroy body cells that are infected by a pathogen or cancer cells.
Helper T cells activate both B and T cells.
B cells produce memory cells and plasma cells. The plasma cells secrete antibodies in prodigious numbers. These will circulate in the blood, and bind and destroy the antigen.

15. B cell immune response Y Y Y Y Y Y Y 10 to 17 days for full response
B cells + antibodies Y
invader (foreign antigen)
tested by B cells
(in blood & lymph)
memory cells
“reserves” Y Y
recognition Y
captured invaders Y
clone
1000s of clone cells
plasma cells
release antibodies Y Y

16. T cell response infected cell or activated macrophage Y Y
killer T cell
activate killer T cells
infected cell
helper T cell
interleukin 2
helper T cell
interleukin 1 or
stimulate B cells & antibodies Y
activated
macrophage
interleukin 2
helper T cell Y

17. Immunological Memory Primary immune response: 1st exposure to antigen
Memory cells:
Secondary immune response: repeat exposure  faster, greater response

18. 1° versus 2° response to disease
Memory B cells allow a rapid, amplified response with future exposure to pathogen

19. B cells that differ in antigen specificity Antigen Antigen receptor
Figure 43.14
B cells that differ in antigen specificity
Antigen
Antigen receptor
Figure Clonal selection.
Antibody
Memory cells
Plasma cells

22. Immune response Y pathogen invasion antigen exposure skin skin
free antigens in blood
antigens on infected cells
macrophages
(APC)
humoral response
cellular response
B cells
helper T cells
T cells
plasma B cells
memory B cells
memory T cells
cytotoxic T cells Y
antibodies

23. Passive immunity: via antibodies in breast milk
Immunizations/vaccines: induce immune memory to nonpathogenic microbe or toxin
Passive immunity: via antibodies in breast milk
HIV: infect Helper T cells
AIDS = severely weakened immune system
Passive Immunity
Occurs when an individual receives antibodies, such as those passed to the fetus across the placenta and to infants via milk.
Active Immunity
Develops naturally in response to an infection; it also develops artificially by immunization. In immunization, a non-pathogenic form of a microbe or part of a microbe elicits an immune response resulting in immunological memory for that microbe.
In localized allergies such as hay fever, IgE antibodies produced after first exposure to an allergen enters the body, it bonds to mast cell-associated IgE molecules, inducing the cell to release histamine and other mediators that cause vascular changes and typical symptoms.

24. Immune system malfunctions
Auto-immune diseases
immune system attacks own molecules & cells
lupus
antibodies against many molecules released by normal breakdown of cells
rheumatoid arthritis
antibodies causing damage to cartilage & bone
diabetes
beta-islet cells of pancreas attacked & destroyed
multiple sclerosis
T cells attack myelin sheath of brain & spinal cord nerves
Allergies
over-reaction to environmental antigens
allergens = proteins on pollen, dust mites, in animal saliva
stimulates release of histamine
Autoimmune disorders:
In each case, the immune system turns against particular molecules of the body, allowing cytotoxic T cells to attack and damage the body’s own healthy cells.
Allergies: hypersensitive responses to harmless antigens

25. Key attributes of immune system
4 attributes that characterize the immune system as a whole
specificity
antigen-antibody specificity
diversity
react to millions of antigens
memory
rapid 2° response
ability to distinguish self vs. non-self
maturation & training process to reduce auto-immune disease

26. Chapter 43 Wrap-Up Define the following terms:
Pathogen
Antigen
Antibody
Allergen
Vaccine
What are lymphocytes? Where do B cells and T cells mature?

27. Ch. 43 Review
What is the difference between innate vs. adaptive immunity?
Contrast the functions of B cells and T cells.
How are antigens recognized by immune system cells?
What are memory cells?
How does HIV affect the immune system?