Disease- conditions that impair normal tissue function
Primary function of Immune System
To provide immunity or resistance to disease
To recognize molecules & act to immobilize, neutralize or destroy them
Immunity involves physiological activities that alert the body to the entrance of foreign materials &
destroy or render them harmless; it is the resistance to specific foreign substances or the toxins
that they produce
Foreign substances: bacteria, viruses, various unicellular/multicellular organisms or organ transplants
Key factor of immunity: the ability to distinguish its own substances (self) from foreign materials (non
self) that enter body & to dispose of them
Infectious disease
Caused by the entrance of an organism into the body & its multiplication there
Can be transmitted to other individuals
Pathogen
Any agent that causes an infectious disease
Must be able to enter host, adhere to host cells, invade/colonize host tissue & inflict damage on
tissue
Disease spread by the pathogen being transferred from one host to another either directly or indirectly
Major types of infectious agents (pathogens)
Bacteria- about 100 bacteria species are pathogens for humans
Bacilli type- legionnaire’s disease, tuberculosis, leprosy
Cocci type- strep throat, staph infections, gonorrhea
Spirilla type- syphilis, trench mouth, black plaque
Viruses- cause disease by disrupting normal cell functions
Fungi- ex. Ringworm, vaginal yeast infection, thrush, athletes foot
Protozoa- acquired through contaminated water/food or bites; ex. Giardiasis, malaria
Helminthes- worms, difficult to treat since drugs also toxic to human cells; ex. Trichinosis,
schistosomiasis
Prions- protein infectious agents; ex. CJD
Pathogens usually produce toxins- chemical substances that harm normal functioning cells
Neurotoxins- powerful toxins that disrupt the transmission of nerve impulses
Some pathogens destroy cells, some burrow into tissue & interfere with normal functions
Mechanisms of disease transmission
Air- in spores, cysts or water droplets
Water- wastes in water supplies can help transmit disease; cold viruses found in nasal secretions
& not saliva
Contact- touching person or an object that person has touched; cold virus can survive for several
hours on surfaces; direct or indirect contact
Arthropods- lice, mosquitoes, ticks, fleas, etc; can penetrate skin of humans & transmit disease
Robert Koch- German doctor who used the scientific method to identify the pathogen responsible for a
particular disease
1. Studied cattle with anthrax & first observed that infected cows had a bacteria in their blood
which he hypothesized caused the disease
2. Tested hypothesis by isolating the bacteria, growing it in laboratory & then injecting it into a
healthy cow
3. Expected the healthy cow to become ill if his hypothesis was correct
4. Healthy cow did become sick & Koch took blood samples to compare the bacteria which he
found to be the same
5. Concluded that the bacteria caused the disease & since then his method of research has
been called Koch’s Postulates
Using Koch’s Postulates, many pathogens have been isolated/identified
Body fights disease with nonspecific or specific defenses
Nonspecific defenses
React to all pathogens in the same way
Primary defense
Skin- overlapping cells of epidermis keep pathogens from entering the body
If it is broken, then b dots form a temporary barrier
Acidic chemicals in sweat, sebum/oil and waxes that are poisonous to some bacteria
Naturally occurring bacteria which help reduce growth of pathogens
Forms a physical/chemical barrier, anatomical barrier, physiological deterrents
Low moisture, low pH, high salinity all help to prevent microorganism growth
Mucus membrane
Trap pathogens so that enzymes can destroy them
Phagocytes
WBC gather & engulf foreign substances
Neutrophils- small WBC with irregular nuclei which ingest small amounts of bacteria
Macrophages- large WBC that engulf hundreds of bacteria
Fever Increase in body temperature which accompanies the action of phagocytes
High fever can kill pathogens but if severe it can cause death itself
An adaptive defense mechanism that accelerates tissue repair
Normally beneficial
Inflammation involves swelling, reddening, increased temperature & pain
Interferon
Protein produced by cells which inhibits the reproduction of viruses
Triggers an enzyme which helps cell recognize virus as foreign
Other Tears/saliva contain enzymes which inhibit growth of bacteria
Stomach acid is chemical barrier against microorganisms entering digestive tract with
food
Cilia keep nasal passages & respiratory tract free of microorganisms
Infections occur when pathogenic microorganisms become established in or on the body
High levels of stress have negative effect on immune system & lower person’s resistance to infection as
does alcoholism, smoking, malnutrition, & lack of sleep
When pathogens successfully invade body & are in process of causing an infection, the body initiates an
immune response
Immunity- resistance to specific foreign agents, such as pathogens or the toxins they release
Important components of immune system
Neutrophils- WBC that move out of bloodstream into surrounding tissues
Lymphocytes- WBC that deal in antibody production; mature into B cells & T cells
Macrophages- large WBC
Antibodies- substances produced by immune system cells to destroy antigens
Antigens- specific proteins carried on microorganisms that identify the microorganism as a foreign
invader to be destroyed; causes your body to develop a specific immune response to pathogen
Specific antibodies produced by the body fit with specific antigens in what is referred to as the “lock &
key” arrangement
Antibody-Antigen reaction causes clumping of cells
Specific defenses
React to specific pathogens- second line of defense
Includes both primary & secondary responses
Primary Immune Response
Defense involving lymphocytes at first exposure to antigen
When the body cells first encounter a particular antigen & lymphocytes are programmed to act
against the specific antigen that has entered the tissue
T cells- attack cells
T lymphocytes originate in bone marrow & then enter thymus where they change so
that they can recognize/destroy foreign materials
Leave thymus & enter lymph nodes where they carry on cellular immunity involving
allergic reactions, rejection of foreign tissue transplants, viruses & certain
cancers
These T cells then seed the thymus, spleen & lymph nodes to begin producing selfreplacing lymphocytes
Must have direct physical contact with pathogen cells
Secrete chemicals to attract macrophages (killer T cells) that dissolve the cell membrane
& ingest the contents to destroy the pathogen; then display antigens from
pathogen on their surface
When they recognize foreign substance, the T cells rapidly divide, some attack the
antigens while others stay in blood as memory T cells to destroy any identical
antigens
Helper T cells stimulate B cell to produce more antibodies; do this by identifying
pathogens & sending out chemical signal molecules to start cell/antibody
mediated responses
B cells
Originate in bone marrow & then enter lymph nodes/spleen where they change into
B cells & memory cells when exposed to a certain pathogen
Produce antibodies immediately to destroy a specific pathogen (bacteria or virus)
This is called humoral immunity since the antibodies are secreted into blood/lymph
Each B cell can only make a single type of antibody
Secondary Immune Response
Response to a subsequent infection by the same pathogen
Quicker than initial response
After pathogen disappears suppressor T cells stop B cells from producing antibodies but B
cells remain in blood
If pathogen appears again the memory B cells & T cells destroy it before it makes the organism
ill
Memory B cells become active B cells that produce antibodies
May take up to week for body to learn how to fight off an unfamiliar microbe
Memory cells recognize pathogen & stimulate production of antibodies
Cell mediated response involves T cells
Responsible for directly destroying body cells that are infected with virus or have
become cancerous
Also for activating other immune cells to be helper T cells
Antibody mediated response involves both T cells & B cells
Responsible for destruction of invading pathogens & elimination of toxins
Occurs when signals activate specific B cells to produce antibodies
Antibodies are proteins called immunoglobulins & are part of the plasma
IgG- occurs in tissue fluid (lymph) & plasma; effective against bacteria, viruses, & toxins by
activating enzymes to attach foreign particles
IgA- found in exocrine gland secretions; effective against viruses that affect respiratory/digestive
systems by causing the viruses to clump together
IgM- naturally occur in blood plasma; reacts with antigens that occur naturally on RBC by
causing them to clump together; ex. A, B, +
IgE- responsible for allergic symptoms
IgD
Ways antibodies destroy antigens
Antibody combines with antigens to render antigen harmless by interfering with its chemical
structure
Attached antibodies cause antigens to clump together in bunches which makes it easier for
macrophages to engulf them
Types of immunity
Naturally acquired active immunity
When a person who has been exposed to a live pathogen develops the disease &
becomes resistant to that pathogen
Artificially acquired active immunity
When a person receives a vaccine & is exposed to a weakened or dead pathogen &
becomes resistant to that pathogen
Can be a shot, liquid or nasal spray
Artificially acquired passive immunity
When a person receives an injection of gamma globulin or anti-serum containing
antibodies so person has immunity for a short time without any immune
response
Wears off over time
Naturally acquired passive immunity
When a fetus receives IgG antibodies from mother or infant receives breast milk
colostrums
Short term immunity for infant so it wears off
Vaccinations help prevent disease by causing immunity & prevent epidemics
Vaccines
Some contain genetically altered or weakened (attenuated) organisms; often give lifelong
immunity; ex. Smallpox, measles, tuberculosis
Some are made of inactivated (killed) whole organisms; may only offer short lived humoral
immunity; need booster doses; ex. Pertusis, cholera, influenza
Some contain toxic products of microorganisms; ex. Tetanus
Reasons adults should update vaccinations
You may be susceptible to a deadly illness
You may not have received natural immunity or vaccination when younger
Childhood vaccinations do not guarantee a lifetime of immunity
Antibiotics
Drugs that fight only bacterial infections- not viral
Used to treat large number of diseases
Penicillin attacks cell wall of bacteria, others disrupt cells vital machinery, others deform DNA
in bacterial cell
Why bacteria becomes resistant to antibiotics- develop way to seal cell wall, use pumps to remove
antibacterial agents that enter cell, develop anti-antibotic enzymes to break compound apart
More you use antibiotics, the more likely you will be infected with resistant bacterial strains
MRSA- methicillin resistant staphylococcus aureus
Strain of staph that have acquired a drug resistance gene that makes it resistant to multiple
antibiotics
Becoming more common
Those at risk- people who take frequent low doses of antibiotics or fail to finish an antibiotic
prescription, people hospitalized or had surgery within past year
Can be treated with newer antibiotics but they are expensive, have side effects & may be hard
to get
Without early treatment, MRSA infections can have fatal consequences
How to protect yourself- proper handwashing that includes duration/scrubbing using warm
water & soap
Some Disorders of the Immune System
Categories of Diseases
Autoimmune
Produced by failure of immune system to recognize/tolerate self-antigens which
results in activation of autoreactive T cells & production of auto-antibodies by
B cells
Causes inflammation & organ damage
Ex. Rheumatoid arthritis, type I diabetes, multiple sclerosis, Graves, lupus, psoriasis
Immune Complex
Formation of free antigen-antibody complexes that activate complement proteins &
promotes inflammation
Can result from infections or the formation of complexes between self antigens &
auto-antibodies
Ex. Rheumatoid arthritis, lupus, hepatitis B
Allergy
10-20% of population has allergies of some sort; tendency can be hereditary but you
can develop allergies at any time
At first a person is exposed to a harmless antigen & its B cells make antibodies against
it showing no allergic reaction but on next exposure the antibodies/antigens
react to produce allergic reaction
Symptoms of allergy depend on where mast cells are located
Lungs- coughing, wheezing, asthma
Skin- hives
Digestive tract- abdominal pain, constipation, diarrhea, nausea, vomiting
Nose, sinuses, eyes, throat- sneezing, sniffling, runny nose, itchy eyes
Immediate Hypersensitivity
Due to abnormal B cell response to an allergen within seconds or minutes
B cells & plasma cells secrete IgE antibodies rather than IgG antibodies
IgE antibodies attach to mast cells & cause mast cells to secrete histamine,
prostaglandin D or leukotrienes
Delayed Hypersensitivity
ex. Contact dermatitis
Symptoms take hours to days to develop (24-72 hr)
Symptoms caused by secretion of lymphokines
Treated by corticosteroids rather than antihistamines
Organ transplants
In US each day 6 people die waiting for an organ & 48 people are added to the transplant list
Commonly transplanted organs- kidneys, liver, heart, lungs, pancreas, cornea
Graft rejection- T cells are activated by antigens on donated tissue & attack to destroy foreign
cells
Use immunosuppressor drugs to reduce sensitivity of immune system
Lyme Disease
Caused by bacteria that is transmitted to humans through tick bites
Symptoms: joint degeneration, chronic fever, prominent bullseye rash
Joint destruction similar to rheumatoid arthritis but other symptoms due to release of
interleukin-1 by activated macrophages
Interleukin-1 exaggerates symptoms associated with primary infection
Treatment: antibiotics & anti-inflammatory drugs
Lupus
90% of those affected are women
3x more common in black women
Affects over ¼ million in US
Cause not known but may be combination of genetics, environmental & hormonal factors
Affects persons of any age but most often strikes young women
Affect joints, skin, kidneys, lungs, heart, blood vessels, nervous system, blood & brain
Causes immune system to attack its own body
Severe joint/muscle pain, extreme exhaustion, fever, skin rashes
No cure & can lead to organ failure & death
AIDS (acquired immune deficiency syndrome)
Caused by viruses (HIV, HTLV-3, LAV)
Virus attacks the cells of immune system by destroying helper T cells
Retrovirus which means it can reverse transcribe RNA into DNA
Lentivirus which means it is slow to act
Transmitted through exchange of body fluids, sharing of drug needles, from mother to fetus
through placenta or mother’s milk
Blood test is used to detect antibodies to HIV to tell whether a person is exposed; once
infected with HIV it takes 2 months to test positive for it; ELISA (enzyme linked
immunosorbent assay) considered 100% accurate but always confirmed with a 2nd
test (Western blot test) before giving results to person
Once virus enters the bloodstream it is recognized as an antigen
Phagocytic WBC engulfs it but is unable to destroy it
T4 cell attacks it next but the virus enters it unharmed; may lie dormant for 10 years
B cell antibodies continue to attack the virus to no avail
When activated it produces many new viruses inside the T4 cells which are released
to infect more T4 cells & stop their ability to fight disease
AIDS patients are susceptible to any disease that comes along; don’t die of AIDS but
are killed by a secondary infection that they can’t fight off
Symptoms: chronic episodes of fever, night sweats, diarrhea, weight loss, fatigue, swollen lymph
glands, skin rashes, dementia, memory loss, partial paralysis, loss of coordination
Progression of disease
HIV positive (1st stage)- blood test shows that person has virus; no symptoms at this
time but can spread disease to others
nd
ARC (2 stage)- early signs include swollen glands, yeast infection & oral lesions, lack
of energy, weight loss, fever & sweats
AIDS (3rd stage)- symptoms include exhaustion, weight loss, lack of appetite, dry cough,
constant fever, diarrhea, secondary infections; fatal within 2 years of onset
Treatment: drugs such as AZT & ddI which disrupt viral reproduction by forming dysfunctional
nuclear material; not a cure; other drugs such as foscarnet & prostrain may improve
survival of patient; group of drugs called nucleoside reverse transcriptase inhibitors or
protease inhibitors; drugs do not get rid of virus- just the signs of it; virus hides out in
the body; many believe that HIV infection will last a lifetime