1. Defenses Against Disease
Chapter 36
Pages

2. What Causes Disease? Microbes - bacteria, protists, fungi, and viruses
Most live in water or the soil
most that live in animal bodies but do not harm them and may be beneficial
When they cause disease, they are pathogens
Most, such as cholera, measles, plague, tetanus, and chicken pox, have been with humans for thousands of years
New, more deadly strains of familiar pathogens are called emerging infectious diseases

3. Emerging Infectious Diseases
Since the early 1980s - HIV, Ebola virus, West Nile virus, SARS, swine flu, bird flu
One strain of the common intestinal bacterium E. coli, which is normally harmless, can cause food poisoning
Some Staphylococcus bacteria that normally cannot penetrate the skin cause severe infections or fatal toxic shock syndrome when they enter the body

4. Defense Against Disease
Three lines of defense
Nonspecific, external barriers
Nonspecific, internal defenses
Specific internal defenses

5. Nonspecific, External Barriers
Prevent microbes from entering the body
Anatomical – skin, cilia, and secretions as tears, saliva, and mucus
Cover external surfaces and line the body cavities that come in contact with the external environment
Surfaces of the respiratory, digestive, and urogenital tracts

6. Nonspecific, Internal Defenses
If the external barriers are breached, the innate immune response, swings into action
Components include:
White blood cells engulf foreign particles or destroy infected cells
Chemicals released by damaged cells and proteins released by white blood cells trigger inflammation and fever
These responses operate regardless of the exact nature of the invader, neutralizing the threat

7. Specific, Internal Defenses
The final line of defense is the adaptive immune response
Immune cells selectively destroy specific invading microbes or toxins and remember the invader
This allows for a rapid response to the invader if it reappears in the future

10. Invertebrate Animals Possess only the first two lines of defense
Lack adaptive immune responses and must rely on the two nonspecific defenses:
External skeletons
Slimy secretions
White blood cells that attack pathogens and secrete proteins to neutralize the invaders or toxins
Defensive proteins, such as lysosome, are similar in vertebrates and invertebrates, suggesting a common ancestor among most of today’s animal species

11. How Nonspecific Defenses Function
The skin and mucous membranes form external barriers
The first line of defense consists of two surfaces with direct exposure to the environment
The skin
The mucous membranes of the digestive, respiratory, and urogenital tracts

12. Skin
The skin and its secretions block entry and provide an inhospitable environment for microbial growth
The outer surface of the skin is dry, dead cells filled with tough proteins that prevent microbes from obtaining the water and nutrients they need
The secretions from sweat and sebaceous glands contain natural antibiotics, such as lactic acid, that inhibit the growth of many bacteria and fungi

13. Mucous Membranes
Antimicrobial secretions, mucus, and ciliary action defend the mucous membranes against microbes
Mucous membrane secretions traps microbes
They contain antibacterial proteins
lysozyme, which kills bacteria by digesting their cell walls
defensin, which makes holes in bacterial plasma membranes
Cilia on the membranes sweep up the mucus, so it is swallowed or coughed or sneezed out of the body

14. Mucous Membranes

15. More Nonspecific Defenses
Stomach – protein digesting enzymes and acidity is lethal
The intestines contain harmless bacteria that secrete substances that destroy invading bacteria or fungi
Urinary tract - slight acidity of urine inhibits bacterial growth
In females, acidic secretions and mucus help protect the vagina
Tears, urination, diarrhea, and vomiting all help to expel invaders
Despite these defenses, many disease-causing microbes enter the body through the mucous membranes or through cuts in the skin

16. Innate Immune Response
Combats invading microbes
When microbes penetrate the skin or mucous membranes, they encounter an array of internal defenses, collectively called innate immunity
Innate immune responses are nonspecific—that is, they attack many different types of microbes rather than targeting particular invaders

17. Three Categories of Innate Immune Response
White blood cells or leukocytes, attack and destroy invading cells or the body’s own cells if they have been infected by viruses
The inflammatory response attracts leukocytes to the site of a wound and walls off the injured area, isolating the infected tissue from the rest of the body
Fever is produced when microbes start an infection in the body, which both slows down microbial reproduction and enhances the body’s own fighting abilities

18. White Blood Cells
Phagocytic leukocytes and natural killer cells destroy invading microbes
Several types of leukocytes or phagocytes ingest foreign invaders and cellular debris
Three important types of phagocytes are:
Macrophages
Neutrophils
Dendritic cells
These cells travel within the bloodstream, move through capillary walls, and patrol the body’s tissues

19. The Attack of the Macrophages

20. Natural Killer Cells
Strike at the body’s cells that have become cancerous or been invaded by viruses
The surfaces of normal body cells display proteins of the major histocompatibility complex (MHC) which identify the cell as “self”
Natural killer cells kill any “nonself” cells they encounter by releasing proteins that make holes in the infected or cancerous cell’s membranes, then secrete enzymes through the holes to kill it

21. Inflammatory Response
The inflammatory response attracts phagocytes to injured or infected tissue
Tissues become warm, red, swollen, and painful
Several functions:
It attracts phagocytes to infected or injured tissue
It promotes blood clotting
It initiates protective behavior by causing pain

22. Inflammatory Response and Mast Cells
Inflammatory response begins when damaged cells release chemicals that cause mast cells, to release histamine
Histamine relaxes the smooth muscle surrounding arterioles, increasing blood flow and causing capillary walls to become leaky
Extra blood flowing through leaky capillaries drives fluid from the blood and into the wounded area, causing redness, warmth, and swelling

23. Chemicals released by wounded cells, mast cells, and by the microbes themselves attract macrophages, neutrophils, and dendritic cells
Consume bacteria, dirt, and cellular debris
Pus, a thick, whitish mixture of dead bacteria, tissue debris, and white blood cells, may accumulate
Other chemicals released by injured cells initiate blood clotting to reduce blood loss and prevent more microbes from entering the blood stream

24. Animation: The Inflammatory Response

25. The Inflammatory Response
dead cell
layer
Tissue damage carries
bacteria into the wound 1
epidermis
Wounded cells
release chemicals (red)
that stimulate mast cells 2
Mast cells release
histamine (blue) 3
dermis
Histamine increases capillary
blood flow and permeability 4
Phagocytes leave
the capillaries and ingest
bacteria and dead cells 5

26. Fever Combats large-scale infections
If invaders breach defenses and mount an infection, they may trigger a fever, which is an important part of the body’s defense against infection
The human thermostat, in the hypothalamus of the brain, is set at 97–99ºF
During an infection, macrophages release endogenous pyrogen that travels to the hypothalamus and raises the thermostat’s set point
Elevated body temperature increases phagocytic activity and slows bacterial reproduction

27. More Fever
Fever also stimulates cells infected by viruses to produce interferon
Travels to other cells and increases their resistance to viral attack; also stimulates natural killer cells
In an experiment, volunteers were infected with a virus and given aspirin or a placebo
Those with aspirin had more viruses in their noses and coughed out more viruses than the controls because fevers in the controls helped reduce the viral infection

28. Adaptive Immune System
When nonspecific mechanisms are breached, the body mounts a specific and coordinated adaptive immune response directed against the specific organism
The adaptive immune response attacks one specific type of microbe, overcomes it, and provides future protection against only that microbe

29. Components of Adaptive Immune Response
Cells of the adaptive immune system are distributed throughout the body, with concentrations of cells in certain locations
It consists of three major components: immune cells, tissues and organs, and secreted proteins

30. Immune Cells
Adaptive immune response is produced by interactions among several white blood cell types
Macrophages, dendritic cells, lymphocytes
The key cellular players are B cells and T cells, which arise from stem cells in the bone marrow
Some of stem cells complete their development in the bone marrow, becoming B (for bone) cells
Others migrate from the marrow to the thymus, where they develop into T (for thymus) cells

31. Tissues and Organs
The cells of the immune system are produced and reside in a variety of locations, including:
vessels of the lymphatic system
the lymph nodes
the thymus
the spleen
patches of specialized connective tissue such as tonsils

32. Immune System Organs
Lymph flows through the lymph nodes which contain macrophages and lymphocytes
The thymus is essential for development of some immune cells
The spleen filters blood, exposing it to white blood cells
The tonsils contain macrophages and other white blood cells that sample microbes entering the body through the mouth, destroying many and starting an adaptive immune response

33. The Lymphatic System Contains Much of the Immune System
thymus
bone marrow
thoracic duct
spleen
lymph vessels
lymph nodes
valve prevents
backflow
lymph node
chambers packed
with white blood cells

34. Secreted Proteins
Leukocytes secrete many different proteins, collectively called cytokines, used for communication between cells
A number of proteins in the blood, called complement, assist the immune system in killing invading microbes
Some cytokines and complement proteins are involved in both the innate and adaptive responses
B cells, a type of leukocyte, produce antibodies that help the immune system recognize and destroy invading microbes

35. Adaptive Immune Response Steps
All adaptive immune responses include the same three steps:
Lymphocytes recognize an invading microbe and distinguish the invader from self
They launch an attack
They retain a memory of the invader that allows them to ward off future infections by the same type of microbe

36. Moving Along…..
To understand how the immune system recognizes invaders and initiates a response, we must answer three questions:
How do lymphocytes recognize foreign cells and molecules?
How can lymphocytes produce specific responses to so many different types of cells and molecules?
How do they avoid mistaking the body’s own cells and molecules for invaders?

37. Recognition of Invaders
The adaptive immune system recognizes invaders’ MHC
Bacteria and humans differ because each contains specific, unique, complex molecules
Large, complex molecules are antigens, because they are “antibody generating” molecules that provoke an immune response

38. How Does the Adaptive Immune System Recognize Invaders?
Antigens are located on the surface of microbes
Sometimes viral antigens become incorporated into plasma membranes of infected cells
Viral or bacterial antigens are also “displayed” on the plasma membranes of dendritic cells and macrophages that engulf them
Other antigens, such as toxins released by bacteria, may be in the blood plasma, lymph, or other extracellular fluids

39. Antibody and T-cell Proteins
Recognize and bind to foreign antigens
Lymphocytes generate two types of proteins that recognize, bind, and help to destroy specific antigens:
Antibody proteins, produced by B cells and their offspring
T-cell receptor proteins, produced by T cells

40. Antibody Structure
Proteins composed of two pairs of peptide chains: one pair of identical large (heavy) chains and one pair of identical small (light) chains
Both chains have a constant region, which is similar in all antibodies of the same type, and a variable region that differs among individual antibodies

41. Antibody Binding Sites are 3D
The variable regions at the arm tips bind to antigens
Each binding site has a unique size, shape and charge so that specific molecules fit in and bind to the antibody
The sites are so specific that each antibody can bind only a few, very similar, antigen molecules

42. Antibody Structure antigen antigen light chain heavy chain
Variable regions form
antigen binding sites
Constant regions are
the same in all antibodies
of a given type

43. Function as Receptors or Effectors
Antibodies function as receptors, binding to specific antigens and eliciting a response
OR as effectors, helping them destroy cells or molecules that bear the antigen

44. How does it work?
As a receptor, the stem anchors the antibody in the plasma membrane of the B cell that produced it, and its two arms stick out from the B cell, sampling the blood and lymph for antigens
When the arm of the antibody encounters an antigen with a compatible chemical structure, it binds to it and initiates a response in the B cell
As effectors, antibodies are secreted into the bloodstream, where they neutralize antigens, destroy microbes that bear antigens, or attract macrophages that engulf the microbes

45. Antibodies Serve as Receptors or Effectors
macrophage
B cell
microbe
antibody
antigen
antibody
microbe
microbe
antigen
(a) Antibody receptor function
(b) Antibody effector function

46. Activated T-cells Trigger an Immune Response
T-cell receptors recognize invaders and help trigger an immune response
T-cell receptors are found on T-cell surfaces
Like antibodies, they consist of peptide chains that form specific antigen binding sites
Unlike antibodies, T-cell receptors are not released into the bloodstream, and do not directly contribute to the destruction of microbes or toxins
T-cell receptors trigger a response in its T cell when the receptor binds an antigen on a cell that has ingested an invading microbe

47. There are millions of them
The immune system recognizes millions of different antigens
The adaptive immune system recognizes and responds to all antigens that are encountered, because B and T cells produce millions of different antibodies and T-cell receptors
How can the body produce so many?

48. Antibody genes are assembled from DNA segments
There are not genes for whole antibodies
B cells have genes that code for parts — constant regions (C), variable regions (V), and regions that connect the two
The constant region in each chain is the same for any antibody of a particular type
Humans have 200 genes for the variable region of heavy chains and 150 genes for the variable region of the light chain

49. Antibody are Built in B-cells
As each B cell develops, it cuts and discards all but one gene of each type, then assembles two unique antibody genes from the genes
A heavy-chain gene - consists of one variable and one constant region
A light-chain gene - consists of one variable and one constant region
Antibodies are produced from composite genes

50. Recombination Produces Antibody Genes
heavy
chain V1 V2 V3 V4
V200 D1 D2
D50 J1 J2 J6 CM CD CG CE CA
light
chain V1 V2 V3 V4
V150 J1 J2 J3 J4 J5 CK
(a) Genes for parts of the heavy chain (top) and light chain (bottom) of antibodies
heavy
chain V2
D11 J4 CG
V87 D8 J1 CG
V111
D40 J1 CG
light
chain
V80 J2 CK
V101 J5 CK V6 J1 CK
Cell 1
Cell 2
Cell 3
(b) Complete antibody genes in three different B cells V2
V87
V111
V80
D11 D8
V80
V101
V101 V6
D40 V6 J4 J1 J2 J2 J5 J5 J1 J1 J1 CK CK CK CK CK CK CG CG CG CG CG CG
Cell 1
Cell 2
Cell 3
(c) Antibodies synthesized by these three B cells

51. How many?
The random assembly of composite antibody genes yields 3 million unique combinations
Further diversity arises because only part of each joining region is actually used in any given antibody
Immunologists estimate that 15 to 20 billion unique antibodies are possible - 2 X 1010
The result is that each B cell produces an antibody that is different from the one produced by every other B cell, except its own daughter cells

52. T-cell Receptor Construction
T-cell receptors are made from different genes, but the process is similar
There are more parts available for constructing T-cell receptor genes, so there may be as many as a quadrillion different possible T-cell receptors!
1015

53. Not Designer Made
Antibodies and T-cell receptors are not tailor-made for antigens
B and T cells do not design antibodies and T-cell receptors to fit invading antigens
Instead, the immune system randomly synthesizes millions of different antibodies and T-cell receptors
Antigens almost always encounter antibodies or T-cell receptors that will bind them

54. How to distinguish self from non-self
The immune system distinguishes self from non-self
Surface of body cells have proteins/polysaccharides that are called the major histocompatibility complex (MHC), unique to each person
If the cells of the immune system bind to the antigens of the MHC, they undergo apoptosis or programmed cell death
The immune system distinguishes self from non-self by retaining only those immune cells that do not respond to the body’s own molecules

55. Regulatory T-cells
Not all self-reactive B and T cells are eliminated in this way
Regulatory T-cells prevent these self-reacting lymphocytes from attacking the body and causing autoimmune disease
A person’s MHC proteins act as foreign antigens in other people’s bodies during organ transplant, donor MHC’s must be similar

56. Two types of Attacks
The adaptive immune system simultaneously launches two types of attack against antigens:
Humoral immunity - B cells and antibodies that they secrete into the blood that attack pathogens outside the body’s cells
Cell-mediated immunity is produced by a type of T cell called the cytotoxic T cell that attacks infected body cells, killing both the cell and any pathogens inside it

57. Immunity takes time to develop
A person may have millions of different antibodies and T-cell receptors, there is only one cell bearing each type of antibody or T-cell receptor
The immune system requires time to be effective because cells recognizing the invader must multiply and differentiate
It takes 1 or 2 weeks to mount a strong immune response after the first exposure to an invading microbe

58. An Effective Immune Response Takes Time

59. Humoral Immunity Produced by antibodies carried in the blood
Each B cell bears unique antibodies on its surface
When an infection occurs, the antibodies borne by a few B cells can bind to antigens on the invader
Antigen–antibody binding causes B cells to divide rapidly by the process of clonal selection, producing a population of “clones” of the original cell

60. Clones or Daughter Cells
The daughter cells differentiate into:
Memory B cells, play an important role in future immunity to the invader
Plasma cells, enlarge and produce a huge quantity of specific antibodies which are released into the bloodstream

61. Animation: B Cell Activation and Differentiation

62. Clonal Selection Among B Cells
antibodies
antigens
Invading antigens
bind to antibodies on
one B cell (dark blue) 1
The B cell “selected”
by the antigen multiplies
rapidly 2
A large clone of
genetically identical
B cells is produced 3
These B cells
differentiate into
plasma cells and
memory B cells 4
plasma cell
memory B
cell
Plasma cells
release antibodies
into the blood 5
endoplasmic
reticulum
antibodies

63. Action of Humoral Antibodies
Multiple modes of action-
Antibodies in the blood combat invading molecules or microbes in three ways:
The circulating antibodies bind to a foreign molecule, virus, or cell and render it harmless by neutralization
Example - an antibody covering the active site of a toxic enzyme in snake venom

64. Antibodies Neutralize Toxins
snake
venom
enzyme
active
site
antibody
Antibodies block the
active site of the toxic
enzymes in snake venom

65. Antibodies coat the surface of invading molecules, viruses, or cells and make it easier for phagocytic cells to destroy them
Macrophages recognize the antibody stems sticking out into the blood, then engulf the antibody-coated invaders and digest them
When antibodies bind to antigens on the surface of a microbe, the antibodies interact with complement proteins that are present in the blood
Some of the complement proteins punch holes in the plasma membranes of the microbe, killing it
Other complement proteins promote phagocytosis

66. Humoral immunity fights invaders that are outside cells
Antibodies cannot enter cells; therefore, the humoral response is effective only against antigens when they are outside of cells, in the blood or extracellular fluid
Viruses are vulnerable when they are outside body cells, but after they enter a body cell, they are safe from antibody attack
Cell-mediated immune reactions are required to fight viruses once they have entered body cells

67. Cell-mediated Immunity
Produced by cytotoxic T cells
Is the body’s primary defense against cells that are cancerous or that have been infected by viruses
Cytotoxic T-cells in the blood bump into an infected body cell that is displaying a viral protein on its surface
The cytotoxic T-cell receptor will bind to the viral protein and squirt proteins onto the surface of the infected cell, punching holes in the cell and killing it
Cancer cells display unusual proteins that the cytotoxic T cells recognize as foreign, and are killed as a result

68. Cell-mediated Immunity in Action

69. Helper T-cells
Helper T cells enhance both humoral and cell-mediated Immunity
B-cells and cytotoxic T-cells are not effective without assistance from helper T cells
Helper T cells have receptors that bind to antigens displayed on the surfaces of dendritic cells or macrophages that have engulfed and digested invading microbes
When its receptor binds an antigen, a helper T cell multiplies rapidly, and its daughter cells differentiate and release cytokinins that stimulate cell division and differentiation in both B cells and cytotoxic T cells

70. Both B-cells and cytotoxic T-cells are most effective against infection when they receive stimulation by cytokinins from helper T cells
Human immunodeficiency virus (HIV) kills helper T cells
Without these cells, the immune system cannot fight off diseases that would otherwise be trivial

71. A Summary of Humoral and Cell-Mediated Immune Response
HUMORAL IMMUNITY
HELPER T CELLS
CELL-MEDIATED IMMUNITY
Targets invaders outside cells (e.g.,
viruses, bacteria, fungi, protists, and
toxins)
Stimulate both humoral and cell-mediated
immunity by releasing cytokines
Targets defective body cells (e.g., infected
cells and cancer cells), transplants
virus
viral
antigen
Viral antigens
presented on the
surfaces of
dendritic cells or
macrophages,
and infected cells
dendritic cell
or macrophage
infected cell
B-cell antibodies bind
to viral antigens and
stimulate the B cells to
divide and differentiate
T-cell receptors bind
to viral antigens
antibody
cytokines
B cell
helper T cell
cytotoxic T cell
Cytokines released by
helper T cells stimulate B
cells and cytotoxic T cells

72. A Summary of Humoral and Cell-Mediated Immune Responses
HUMORAL IMMUNITY
HELPER T CELLS
CELL-MEDIATED IMMUNITY
Targets invaders outside cells (e.g.,
viruses, bacteria, fungi, protists, and
toxins)
Stimulate both humoral and cell-mediated
immunity by releasing cytokines
Targets defective body cells (e.g., infected
cells and cancer cells), transplants
antibody
cytokines
B cell
helper T cell
cytotoxic T cell
Cytokines released by
helper T cells stimulate B
cells and cytotoxic T cells
memory
helper
T cell
memory
cytotoxic
T cell
plasma
cell
memory B cell
cytotoxic T cell
infected
cell
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

74. How Does the Adaptive Immune System Remember?
After recovering from a disease, you remain immune to that particular microbe, perhaps a lifetime
Some of the daughter cells of the original B cells, cytotoxic T cells, and helper T cells that responded to the original infection differentiate into memory B cells and memory T cells and survive for many years
If the body is reinvaded by the same type of microbe, the memory cells recognize the invader and mount an immune response

75. Animation: Humoral Versus Cell-Mediated Immunity

76. Memory Cells
Memory B cells rapidly produce a clone of plasma cells, secreting antibodies that combat this second invasion
Memory T cells produce clones of either helper T cells or cytotoxic T cells specific for the “remembered” invader
Each memory cell responds so fast and so largely in a second infection, the body fends off the attack before the person suffers any symptoms—they have become immune
Acquired immunity confers long-lasting protection against many diseases such as small pox, measles, mumps, and chicken pox

77. Animation: Memory B Cells

78. Acquired Immunity

79. Drug Therapy Antibiotics slow down microbial reproduction
Antibiotics - drugs that combat infection by slowing down the multiplication of bacteria
The occasional mutant microbe that is resistant to an antibiotic will pass its genes for resistance to its offspring, which results in many antibiotics becoming ineffective
Antibiotics are not effective against viruses
Drugs are available that target different stages of the viral cycle of infection, and are used to treat HIV, severe herpes, and in some cases, the flu virus

80. Vaccines and Immunity
Vaccinations stimulate the development of memory cells and future immunity against disease
A vaccine stimulates an immune response by exposing a person to a pathogen’s antigens
May consist of weakened or killed microbes, or some of the pathogen’s antigens
Exposure to these antigens results in the body producing memory cells that confer immunity against similar microbes

81. Allergies Misdirected immune response
Allergies are immune reactions to harmless substances that are treated (by the body) as if they were pathogens
Common allergies include pollen, mold spores, bee or wasp venoms, and some foods such as milk, eggs, fish, wheat, tree nuts, and peanuts

82. Immune System Malfunctions
All allergic reaction begins when allergy-causing antigens, called allergens, enter the body and bind to “allergy antibodies” on a special type of B cell
This B cell proliferates, producing plasma cells that pour out allergy antibodies into the plasma
The antibodies attach to mast cells, mostly in the respiratory and digestive tracts

83. Allergies
If allergens bind to these attached antibodies, they trigger the release of histamine, which causes leaky capillaries and other symptoms of inflammation
In the respiratory tract, histamine increases mucous secretions and results in symptoms
Food allergies may cause intestinal cramps and diarrhea; some reactions are so strong that the airways may completely close, causing death by suffocation

84. An Allergic Reaction to Pollen
First exposure to pollen
(yellow) stimulates B cells to
produce “allergy” plasma cells 1
Plasma cells
produce allergy
antibodies 2
mast
cell
plasma
cell
Allergy antibodies
bind to mast cells 3
Reexposure to pollen results
in pollen binding to allergy
antibodies on mast cells 4
Binding of pollen stimulates mast
cells to release histamine (blue),
triggering the inflammatory response 5

85. Autoimmune Disease An immune response against the body’s own molecules
Occasionally, our immune system produces “anti-self” antibodies
The result is an autoimmune disease in which the immune system attacks a component of one’s own body, such as a type of anemia where antibodies destroy a person’s red blood cells
Type 1 diabetes begins when the immune system attacks insulin-secreting cells of the pancreas
Other autoimmune diseases include rheumatoid arthritis, multiple sclerosis, and systemic lupus

86. Therapy for Autoimmune Disease
No known cures
Replacement therapy can alleviate the symptoms— for example, by giving insulin to diabetics or blood transfusions to anemia victims
The autoimmune response can be reduced with drugs that suppress the immune response
This course of action also reduces responses to the everyday assaults of disease microbes, so the therapy has drawbacks

87. Immune Deficiency Diseases
Occur when the body cannot mount an effective immune response
There are two disorders in which the immune system cannot combat routine infections:
Severe combined immune deficiency (SCID), a group of genetic defects in which few or no immune cells are formed
Acquired immune deficiency syndrome (AIDS), where a viral infection destroys a formerly functional immune system

88. HIV Causes AIDS

89. SCID Severe combined immune deficiency is inherited
A child with severe combined immune deficiency (SCID) may survive the first few months of postnatal life, protected by antibodies acquired from the mother during pregnancy
Once these antibodies are lost, common infections can prove fatal because the child lacking an immune system cannot generate an effective immune response
A form of therapy is to transplant bone marrow from a healthy donor into the child to provide enough immune cells to confer normal immune responses

90. AIDS Acquired immune deficiency disease
The most common immune deficiency disease
AIDS is caused by human immunodeficiency viruses (HIV) that infect and destroy helper T cells, stimulating both the cell-mediated and humoral immune responses
AIDS does not kill people directly, but victims become increasingly susceptible to other diseases as their helper T-cell populations decline

91. How HIV is spread
Because HIV cannot survive for long outside the body, it is transmitted only by the direct contact of broken skin or mucous membranes with virus-laden body fluids, including blood, semen, vaginal secretions, and breast milk
HIV is spread by sexual activity, sharing needles among intravenous drug users, or through blood transfusions
HIV enters a helper T cell and hijacks the cell’s metabolic machinery, forcing it to make more viruses which then emerge, taking an outer coating of T-cell membrane with them

92. Early in the infection, as the immune system fights the virus, the victim may develop a fever, rash, muscle aches, headaches, and enlarged lymph nodes
Over time helper T cell levels drop, severely weakening immune response
As HIV levels increase, they kill more helper T cells and the person is more succeptible to other infections

93. Animation: HIV- The AIDS Virus

94. Several drugs can slow down the replication of HIV and thereby slow the progress of AIDS; unfortunately, HIV can mutate into forms that are resistant to the drugs
Some HIV-positive individuals receiving the best medical care often live out a normal life span
The best solution would be to develop a vaccine
This is a challenge, HIV disables the immune response that a vaccine depends on
HIV has a high mutation rate, perhaps a 1000X times faster than flu viruses
Lone infected individuals may harbor different strains of HIV in their blood and semen because of mutations that occurred within their bodies

95. Immunity and Cancer
Cancer will kill more than 500,000 people in the United States this year
Approximately 40% of U.S. citizens will eventually contract some form of cancer
Triggered by environmental factors such as UV radiation, smoking, faulty genes, mistakes during cell division, and viruses
These triggers produce cancer by sabotaging the mechanisms that normally control the growth of the body’s own cells

96. Immune System recognizes most cancer cells as foreign
Cancer cells are self and the immune response usually does not respond to self
The process that causes a cell to become cancerous leads to slightly different proteins appearing on their surfaces
Natural killer cells and cytotoxic T cells encounter these new proteins, recognize them as non-self antigens, and destroy the cancer cells
Some cancer cells do not bear antigens that allow the immune system to recognize them as foreign or, as in leukemia, suppress the immune system

97. Vaccination can prevent some cancers
Some cancers are caused by viruses - including some cancers of the liver, mouth, throat, penis; some types of leukemia; and cervical cancers
Two vaccines are available that help prevent certain cancers:
Hepatitis B, which reduces the risk of liver cancer
Human papilloma virus, which cause most cases of cervical cancer

98. Some “treatment vaccines” can cure certain cancers by providing a patient with antigens commonly found on cells of the type of cancer that the patient has, often enhanced in various ways to boost the patient’s immune response against the cancer
Current trials of this vaccine against prostate cancer and melanoma are in progress
Other treatment vaccines consist of antigens from a patient’s own tumor cells, often enhanced to stimulate a stronger immune response

99. Another approach is to take antigen-presenting dendritic cells from a patient, expose them to antigens from cancer cells, and force them to multiply rapidly in culture
The resulting daughter cells are then injected back into the patient
This large number of activated dendritic cells should stimulate the patient’s own anticancer immune response

100. Medical Treatments for Cancer
Most depend on selectively killing cancerous cells
Attempts to eliminate cancer mostly focus on surgery, radiation, and chemotherapy
Surgically removing the tumor is the first step in treating many cancers, but it can be difficult to remove all the cancerous tissue
Tumors can be treated with radiation, which can destroy even microscopic clusters of cancer cells by disrupting their DNA, preventing cell division
Neither surgery nor radiation is effective against cancer that has spread throughout the body

101. Chemotherapy Drugs Commonly used to supplement surgery or radiation
Attack the machinery of cell division, so they are somewhat selective for cancer cells, which divide more faster than normal cells
Chemotherapy also kills some healthy, dividing cells
Damage to dividing cells in patient’s hair follicles and intestinal lining by chemotherapy produces its well-known side effects of hair loss, nausea, and vomiting