Microbiology – Introduction to the Immune System and
I. Introduction to the Immune System
A. The ability to ward off the pathogens that produce disease is called
B. Lack of resistance is called __________________________.
C. Resistance to disease can be grouped into two broad areas.
1. Nonspecific immunity – the body’s first line of defense.
a. Mechanisms that provide general protection against
invasion by a wide range of pathogens.
b. This resistance is innate (unlearned).
c. It responds the same to everything – bacteria, viruses,
parasitic worms, etc. are all treated the same.
d. This resistance never learns. It responds the same on
the 1st through the 100th exposures to the same pathogen.
1) __________________________ – disease-
2. Specific immunity
a. Dependent upon lymphocytes (a type WBC or
leukocyte found in the lymphatic system)
b. Specific lymphocytes and specific antibodies respond to
1) __________________________ – anything that
elicits a specific immune response.
a) Antigens are usually foreign (nonself)
b) Antigens are usually macromolecules such
as proteins and polysaccharides.
c. The specific immune system learns through exposure to
1) Memory responses occur much more quickly upon
2nd and subsequent exposures to that antigen.
2) Usually responds upon re-exposure before
symptoms of disease set in
D. The body system that carries out specific immune responses is the
II. Basic terminology & definitions
A. __________________________ – any change from a state of good
health. Classes of diseases include…
1. Hereditary diseases – e.g. cystic fibrosis; many genetic
mutations in a certain gene cause this disease.
2. Dietary diseases – vitamin, mineral, protein deficiencies;
3. Environmental diseases – smog/air pollution; lung diseases.
Also includes poverty & host (child) abuse leading to mental illness.
4. Microbial induced diseases
a. Infectious diseases – diseases caused by the growth of a
microbe in/on a host.
b. Toxin induced diseases – disease caused by toxins
produced by microbes, in which case the living microbe
does not have to be present in the affected host.
B. ____________________________________ – the organism (species
or strain) responsible for producing a disease
C. __________________________ – an association between 2+ different
organisms that has evolved to a state where they are generally found in
association with each other under natural condition & where one or both
members of the association benefits from the relationship
1. e.g. E. coli in humans
D. __________________________________ – the organisms normal
found on or in a healthy host most of the time
E. __________________________ – a type of symbiosis where an
organism lives on or in a host & obtains nutrients from that host.
1. A parasite does not necessarily harm the host & in some cases
their presence is even beneficial to the host.
a. e.g. Tiny mites live in our eyebrows. These insects
apparently life off the dead skin cells, oil, & excretions form
our sweat glands & they do us no apparent harm or good.
2. However, in most cases only one of the associates truly benefits
from the association.
a. e.g. head lice, fleas on your pets, pin worms, etc.
F. __________________________ – a parasite that causes a disease in
G. __________________________ – Growth on or in a host. This
growth may or may not harm the host.
H. __________________________ – An infection with a larger than
1. e.g. parasitic worms, pubic & body louse, etc.
I. Mutualistic symbiosis – a situation where 2+ species live in a situation
where both benefit from the association.
1. In some cases, one or more of the symbiotic species are totally
dependent on the other.
J. __________________________ – the ability to cause a disease.
Many organisms are pathogenic under certain conditions (e.g. the normal
bacteria in our gut, which usually do us no harm, can become highly
pathogenic under specific circumstances like getting shot or stabbed in the
gut), but some are almost always pathogenic (e.g. chicken pox & flu).
K. __________________________ – the degree of pathogenicity. A
virulent organism is one that usually produces a violent & severe disease,
often leading to death.
1. Chickenpox & the common cold virus, while clearly pathogens,
are not considered to be virulent.
2. Yersinia pestis, the etiological agent of bubonic plague, & the
Ebola virus are both considered to be virulent.
L. ______________________________________ – a mutant of a virulent
pathogen that has lost its ability to produce a disease.
1. These organisms may be used in the production of vaccines
M. _____________________________________ – a normal flora
organism that normally is not a pathogen, but that becomes pathogenic
when inoculated into the wrong place or when the host becomes
weakened & immunologically compromised.
1. Immunologically compromised – a host whose immune system
is not working to its full capacity & thus the host’s defenses are
a. People undergoing chemotherapy for cancer are
immunologically compromised because the drugs harm the
b. People who are under stress often have a lowered
immunity & are more susceptible to catching infections.
III. Introduction to the Blood – since many of the immune responses involve
cells of the blood, a brief introduction to the blood is needed.
A. Whole blood consists of…
1. Living cells - called formed elements that are suspended in the
2. Nonliving fluid matrix - __________________________.
B. Blood Plasma is composed of…
1. 90% H2O
2. 8% plasma proteins (mostly produced by the liver)
a. These proteins include immunoglobulins – (a.k.a.
antibodies) made by plasma cells (a special B cell, which is
a type of WBC)
3. 2% solutes that include nutrients, gasses, hormones, wastes,
products of cell activity, and ions.
C. Formed Elements of Blood
1. __________________________ (a.k.a. red blood corpuscles or
a. What is the
function of RBC’s? Transport O2
b. They are small cells that are biconcave in shape.
c. They lack nuclei and most organelles
1) Therefore, they cannot divide
2) They also can’t make new proteins or repair
3) Their average life span is 120 days
d. RBC contain mostly hemoglobin
1) Hemoglobin (Hg) is an oxygen-binding pigment
a) Hg is responsible for the transport of most
of the oxygen in the blood.
b) A Hg molecule consists of 4 globin proteins
bound to 4 red, iron-containing heme pigments.
i) Heme portion binds oxygen
ii) Globin portion binds carbon dioxide
2. __________________________ (a.k.a. white blood cells or
a. Unlike RBC’s, WBC’s have nuclei and are complete cells
1) WBC’s are usually short-lived and make up less
than 1% of the total blood volume.
2) They are critical to our defense against disease.
3) How do they move?
a) amoeboid movement: using pseudopodia
b) diapedesis: squeeze out between cells of
c) positive chemotaxis: attracted to stimuli
d) phagocytosis: "cell eating"
b. Granulocytes are large WBC’s with lobed nuclei and
granules in their cytoplasm; all are phagocytic
1) __________________________ are the most
a) They are chemically attracted to sites of
b) They are active phagocytes (digesting
c) Light pink to blue-black with 2 or more lobes
i) prostaglandins (increase permeability)
ii) leukotrienes (attract other
2) __________________________ have red
a) They are relatively uncommon and attack
b) They have been implicated in allergic
3) __________________________: large, granular
cells with purple to blue nuclei
a) Release histamine and heparin that
b) Least numerous WBC
c. Agranulocytes lack visibly staining granules
1) __________________________ have large oval
or kidney bean shaped nucleus
a) They have amoeboid movement
b) They become macrophages, eating debris
c) They attract WBC's and fibroblasts to wall
d) They activate T cells
2) __________________________ have large nuclei,
they are the size of a RBC with a thin halo of
a) __________________________: cellular
immunity. They directly attack viral-infected
and tumor cells.
1) cytotoxic T cells: destroy foreign
cells in tissues
2) helper T and suppressor T cells:
stimulate or inhibit the activities of other
b) __________________________: humoral
1) They become plasma cells, and
produce antibodies (a.k.a.
2) antibodies bind to foreign (non-self)
antigens (surface proteins), marking the
invader for destruction by the immune
c) A 3rd, and less common, lymphocyte is the
large, granular ________________________
1) They are nonspecific cells that
interact with virus-infected cells & some
other abnormal cells, such as cancerous
2) NK’s release ___________________
(a hole-forming protein) that kills the
targeted, abnormal body cell
D. __________________________ (a.k.a. thrombocytes)
1. Platelets are not complete cells, but fragments of large cells
a. Formation of platelets involves repeated mitoses of
megakaryocytes without cytokinesis.
2. Platelets are critical to the clotting process, forming the
temporary seal when a blood vessel breaks.
1. Drains interstitial fluid
a. The circulatory system leaks plasma into the body tissues
b. This fluid becomes interstitial fluid
c. Interstitial fluid is drained from body tissues by the
2. Returns leaked plasma proteins to the blood
3. Transports dietary fats, and
4. Protects against invasion by nonspecific defenses and specific
B. The lymphatic system consists of…
1. A fluid called __________________________
a. Lymph is formed when interstitial fluid drains into
2. Lymph capillaries merge to form larger vessels, called
a. Lymph vessels convey lymph into and out of structures
called lymph nodes.
3. __________________________ are encapsulated oval
structures composed of specialized reticular tissue containing large
numbers of lymphocytes.
a. Lymph enters a node
b. Within the node, damaged cells, microorganisms, and
foreign substances are filtered from the lymph.
1) Macrophages destroy some foreign substances by
2) Lymphocytes bring about the destruction of others
by specific immune responses.
c. Lymph exits the node
d. Lymph nodes are also the site of proliferation of plasma
cells and T cells.
4. This “cleaned” lymph is then returned to the circulatory system
where it becomes part of the plasma
V. NONSPECIFIC RESISTANCE TO DISEASE
A. Nonspecific Defense Mechanisms
1. Physical barriers, such as skin and mucous membranes
2. Secretions, such as mucus, saliva, urine that flush out microbes
3. Normal floral bacteria
4. Antimicrobial substances in body fluids and phagocytes
6. Physiological defenses, such as fever
B. First Line of Defense: Surface Barriers – the skin and mucous
1. Mechanical protection includes the intact epidermis layer of the
skin, mucous membranes, the lacrimal apparatus, saliva, mucus,
cilia, the epiglottis, and the flow of urine. Defecation and vomiting
also may be considered mechanical processes that expel microbes.
2. Chemical protection is localized on the skin, in loose connective
tissue, stomach, and vagina.
a. The skin produces sebum, which has a low pH due to the
presence of unsaturated fatty acids and lactic acid.
b. Lysozyme is an enzyme component of sweat that also
has antimicrobial properties.
c. Gastric juice renders the stomach nearly sterile because
its low pH (1.5-3.0) kills many bacteria and destroys most of
their toxins; vaginal secretions also are slightly acidic.
d. Cells are capable of killing pathogens using a
e. Cells can produce an iron-binding protein, lactoferrin,
that makes Fe unavailable to pathogens. This limits the
replication of many pathogens.
3. Competition by normal flora prevents infection with many
1. The second line of defense involves…
a. Internal antimicrobial proteins
b. Phagocytic and natural killer cells
a. Cytokines are important to …
1) the development of the inflammatory response
2) the development of fever
3) the development of specific immune responses.
2. __________________________ (IFNs) are produced and
released by body cells infected with viruses.
a. IFNs diffuse to uninfected neighboring cells and binds to
b. Neighboring, uninfected cells, are induced to synthesize
antiviral proteins that interfere with or inhibit viral replication.
c. Other effects of IFNs…
1) Enhance the activity of phagocytes and natural
killer (NK) cells
2) Inhibit cell growth
3) Suppress tumor formation
3. __________________________ – 18+ different cytokines
produced by leukocytes. Their functions include…
a. Inducing fever
b. Signaling for the release of PMN’s (neutrophils) from
c. Attracting WBC’s (leukocytes) to areas of inflammation
d. Inducing the proliferation of lymphocytes (T and B cells)
4. _______________________________________ – direct
immature cells into appropriate maturation pathways to produce
needed cell lines.
a. Myeloid stem cells (in bone marrow) can be directed to
become mature granulocytes (basophils, eosinophils, or
neutrophils) and monocytes (which can become
macrophages and dendritic cells).
b. Lympoid stem cells (in lymph nodes) can be directed to
become mature lymphocytes (T cells, B cells, and natural
c. Megakaryoblasts will be directed to become
megakaryocytes which will become the platelets.
a. Mast cell granules store tumor necrosis factor alpha. Its
1) Induces fever
2) Recruits neutrophils to areas of inflammation
3) It is also antiviral
1. Natural killer (NK) cells are lymphocytes that lack the
membrane molecules that identify T and B cells.
a. NKs are able to lyse and kill cancer cells and virally
infected cells before the adaptive immune system has been
b. NK cells can release perforins.
1) Perforins insert into the plasma membrane of cells,
causing them to become leaky.
2) Cytolysis (death) occurs
c. NKs can also bind to target cells and inflict damage by
2. Phagocytes confront microorganisms that breach the external
a. __________________________ occurs in several steps:
1) chemotaxis – the phagocyte is attracted to nonself
antigens exposed on bacteria surfaces or virally
infected body cells
3) ingestion through endocytosis
4) fusion of resulting vacuole with lysosome produces
5) killing within phagolysosome due to lethal
oxidants contained within the lysosome
6) breakdown of dead materials
a) The phagocyte will present the resulting,
processed nonself antigens together with self
antigens to alert the specific immune system to
7) exocytosis of the resulting debris.
b. Phagocytic cells include…
1) Macrophages - the main phagocytes of the body.
2) Neutrophils - the first responders
a) They become phagocytic when they
encounter infectious material.
3) Eosinophils which are weakly phagocytic
a) They are important in defending the body
against parasitic worms.
4) Mast cells – these cells look like macrophages,
but are in the body tissues, not the circulatory
a) They have the ability to bind with, ingest,
and kill a wide range of bacteria.
F. __________________________ occurs any time the body tissues are
injured by physical trauma, intense heat, irritating chemicals, or infection
by viruses, fungi, or bacteria.
1. The four cardinal signs of acute inflammation are…
2. Substances that contribute to inflammation are histamines,
kinins, prostaglandins, leukotrienes, and complement.
3. The three basic stages of inflammation are…
a. __________________________ - chemicals cause
dilation of surrounding blood vessels. This leads to
increased blood flow to the area increased permeability.
1) Vasodilation results in clotting factors and
antibodies leaving the plasma to enter the tissues.
b. Phagocyte migration - soon after inflammation begins,
the damaged site is invaded by neutrophils and
1) Phagocytes engulf damaged tissue and microbes
2) Eventually, the phagocytes die.
3) This forms a pocket of dead phagocytes and
damaged tissue and fluid called pus.
4) Pus must drain out of the body or it accumulates in
a confined space, causing an abscess.
c. Tissue repair.
G. Physiological Changes contribute to body defense
1. Fever - an abnormally high body temperature.
a. Fever is a systemic response to infection from bacteria
(and their toxins) and viruses.
b. Fever is caused by pyrogens that act on the
hypothalamus to increase body temperature.
c. The high body temperature inhibits some microbial
d. Fever speeds up body reactions that aid repair.
2. Changes in Iron Availability. Iron is made less available to
Microorganisms, hindering their replication.
3. Changes in Protein and Carbohydrate Metabolism. Host
protein and carbohydrate metabolism is increased to meet the
demands of the active immune response.
VI. Nonspecific defense mechanisms of the body’s weak spots – where our
fragile body is actually open to the cold, cruel microbial world around us.
A. Skin – tough, dry, salty, oily, rich in fatty acids & urea, low in nutrients
(lots of dead, empty cells), & thick.
1. Sweat glands secrete a mixture of salt & fatty acids that inhibit
2. The skin is host of normal flora organisms that are antagonistic
to potential pathogens.
a. Normal flora eat up potential nutrients on the skin surface
b. To wash or not to wash? Washing our skin helps to
remove transient (potentially disease-causing) organisms.
1) However, our obsession with cleanliness also
washes away the acid mantle of our skin that helps
prevent infection with disease-causing organisms.
(Cure – weak solution of apple cider vinegar sprayed
on after bath or shower.)
2) Further, the use of antimicrobial soaps kills much
of the good, normal flora & can help produce
antimicrobial resistant “super bugs”.
B. Mouth & Gastrointestinal tract
1. The mouth – harbors a host of microbes that live more or less
permanently, on the inside surface & in the nooks & crannies of the
mouth tissues. These microbes are symbiotic & usually do us little
harm as long as we remain healthy.
a. Some of these species do us harm by contributing to the
production of cavities, but many are beneficial because they
out-compete harmful microbes that otherwise would live in
our oral cavity & do us harm in the process.
b. Mouth microbes have evolved elaborate systems for
sticking to things (e.g. pili)
c. In addition, there is a continuous flow of fluid (saliva)
through the mouth which flushes loose microbes into the
2. The stomach – contains a strong acid, hydrochloric acid. Many
microbes are killed by this acidic environment & digested by the
proteolytic enzymes in the digestive system.
a. It is important to thoroughly chew food in order to expose
potential pathogens to this acid bath.
b. Some bacteria can live & thrive in this low pH
environment. E.g. Helicobbacter pylori, the cause of
stomach ulcers & stomach cancers. This needs to be taken
into account when treating stomach ulcers.
3. The small intestine – full of digestive enzymes & detergents
(bile) that agreeably digest microbes as well as hamburger & pizza.
a. Even though the small intestine may be full of nutrients,
the absorption system of the healthy body is so efficient that
these nutrients are absorbed from the intestine so rapidly
that residential microbes have little to live on.
b. Further, the intestines are anaerobic, so obligate aerobes
are unable to grow there even if they should survive the trip
through the stomach.
4. The large intestine – is a different story. It collects & processes
undigested material that passes through the small intestine.
Bacteria, including some very nasty potential pathogens, grow
robustly on this debris & yet they rarely manage to invade the body.
a. The wall of the large intestine is coated with a protective
mucous layer that separates the contents from direct contact
with the cells lining the large intestine.
b. Epithelial cells of the mucosal epithelium lining body
channels that sometimes communicate with air (e.g. the gut,
lungs, urogenital tract, etc.) secrete antibiotic peptides
(defensins) that kill bacteria by membrane disruption.
c. The normal flora of the large intestine evolved to live on
the available food supply in the anaerobic conditions found
there. Feces are approximately 40% bacteria by weight.
1) One problem with antibiotics taken by mouth is
that they may upset the natural microbial balance &
allow unusual microbes to establish themselves in our
mouth, intestinal tract, or bowel. This often results in
intestinal problems (e.g. excess gas & diarrhea) until
the original mix of microbes is again established.
C. Genitourinary Tract – this region of the body is a rich source of
infection for obvious reasons.
1. The urinary tract
a. The urine is a good nutrient for many microbes.
b. The kidneys provide numerous nooks and crannies
where microbes can hide form the body’s defense systems.
c. Sexual activity significantly increases the exposure to
d. The efficiency of our nonspecific immune system is
shown by the fact that we don’t suffer more urogenital
infections than we do.
1) Microbes are prevented from reaching the bladder
& kidneys mainly due to the vigorous flushing of urine
out of the body through the urethra.
2) Because the urethra in women is shorter than in
men, bladder infections are more common in women.
3) People subject to frequent bladder infections
should drink lots of water & should urinate soon after
sex to wash out potential pathogens that may have
entered the urethra during intercourse.
2. The vaginal area
a. Normally the vagina is acidic due to the growth of
lactobacilli that produce lactic & acetic acid. Also there is a
continuous outward flow of mucus that expels microbes from
the vagina. Further, the entry to the reproductive organs is
blocked by a mucous plug much of the time.
b. However, the vaginal lining is thin & easily damaged by
unsuitable physical activity, including the improper use of
sanitary napkins. Its rich blood supply makes it an easy
entry point for pathogens.
c. Damage to the vaginal lining is a major reason why
women are more likely to become infected with HIV from
3. The anal area
a. It is continually exposed to fecal microbes that should not
be introduced into the blood or tissue.
b. Such introduction is usually prevented by a mucous
covering over the delicate anal membrane lining.
c. Anal penetration by foreign objects is very dangerous
because these delicate membranes are easily torn & begin
bleeding. This explains why sexually transmitted diseases
(STD’s) are so readily contracted through anal intercourse.
D. Respiratory tract – the lungs, with their large surface area &
numerous small chambers, offer a rich source of nutrients & a great
potential for concealing pathogens.
1. Everyday we breathe in hundreds of liters of air contaminated
with dust, pollen, and microbes, yet rarely do we get lung infections.
2. The nose is designed so that the turbulent flow of air throws
particulate matter onto the sticky mucous lining where much of it
3. Particles trapped in mucous in the nose & lungs are moved by
the beating of cilia that line the air passages into the throat where
they are swallowed.
4. Cells lining the air passages also secrete defensins that punch
holes in the membranes of many bacteria & destroy them.
5. Sneezing & coughing expels material out of the air passages.