Defenses Against Disease

Chapter 36

Pages 691-712

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

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

Defense Against Disease Three lines of defense

Nonspecific, external barriers

Nonspecific, internal defenses

Specific internal defenses

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

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

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

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

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

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

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

Mucous Membranes

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

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

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

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

The Attack of the Macrophages

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

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

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

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

Animation: The Inflammatory Response

The Inflammatory Response

Phagocytes leavethe capillaries and ingestbacteria and dead cells

5

Tissue damage carriesbacteria into the wound1

Wounded cellsrelease chemicals (red)that stimulate mast cells

2

Mast cells releasehistamine (blue)3

Histamine increases capillaryblood flow and permeability4

dead celllayer

epidermis

dermis

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

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

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

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

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

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

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

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

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

Adaptive Immune Response Steps All adaptive immune responses include the

same three steps:

1. Lymphocytes recognize an invading microbe and distinguish the invader from self

2. They launch an attack

3. They retain a memory of the invader that allows them to ward off future infections by the same type of microbe

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

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

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

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

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

light chain heavy

chain

antigen

Variable regions formantigen binding sites

Constant regions arethe same in all antibodiesof a given type

antigen

Antibody Structure

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

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

Antibodies Serve as Receptors or Effectors

(a) Antibody receptor function

B cell

antibody

antigen(b) Antibody effector function

antibody

antigen

macrophage

microbe

microbe

mic

robe

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

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?

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

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

V1 V2 V3 V4 V200 D1 D2 D50 J1 J2 J6 CM CD CG CE CA

V1 V2 V3 V4

(a) Genes for parts of the heavy chain (top) and light chain (bottom) of antibodies

(b) Complete antibody genes in three different B cells

(c) Antibodies synthesized by these three B cells

heavychain

lightchain

heavychain

lightchain

V150 J1 J2 J3 J4 J5 CK

V2

V2

D11

D11

J4

J4

V80

V80 V80

J2

J2 J2

CK

CK CK

V101 J5 CK V6 J1 CK

CG V87 D8 J1 CG V111 D40 J1 CG

Cell 1

V87

D8

J1

V101 V101

J5 J5

CK CK

Cell 2

V111

D40

J1

V6 V6

J1 J1

CK CK

Cell 3

Cell 1 Cell 2 Cell 3

CG CG CG CG CG CG

Recombination Produces Antibody Genes

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

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

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

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

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

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

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

An Effective Immune Response Takes Time

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

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

Animation: B Cell Activation and Differentiation

Clonal Selection Among B Cells 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

Action of Humoral Antibodies Multiple modes of action-

Antibodies in the blood combat invading molecules or microbes in three ways:

1. 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

Antibodies block theactive site of the toxicenzymes in snake venom

snake venomenzyme active

site

antibody

Antibodies Neutralize Toxins

2. 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

3. 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

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

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

Cell-mediated Immunity in Action

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

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

A Summary of Humoral and Cell-Mediated Immune Response

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

A Summary of Humoral and Cell-Mediated Immune Responses

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

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

Animation: Humoral Versus Cell-Mediated Immunity

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

Animation: Memory B Cells

Acquired Immunity

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

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

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

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

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

An Allergic Reaction to Pollen

Plasma cellsproduce allergyantibodies

2

First exposure to pollen(yellow) stimulates B cells toproduce “allergy” plasma cells

1

Allergy antibodiesbind to mast cells3

Reexposure to pollen resultsin pollen binding to allergy antibodies on mast cells

4

plasmacell

mastcell

Binding of pollen stimulates mastcells to release histamine (blue),triggering the inflammatory response

5

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

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

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

HIV Causes AIDS

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

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

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

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

Animation: HIV- The AIDS Virus

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

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

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

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

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

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

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

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