MBIO: module 3

the immmune response

how is the innate immune system divided?

* physical barriers (ex: hair, cilia, skin, mucus membranes, digestive enzymes in mouth, stomach acid)
* internal defenses (inflammatory response, complement proteins, phagocytic cells, NK cells)

what is the innate immune system?

the immune system that we are born with or develop from birth

what is the adaptive immune system?

antibodies and the humoral response

cell-mediated response

memory response

first line of defence:

1. mechanical and physical barriers to pathogen invasion
2. cellular and chemical factors
3. microbial antagonism by our microflora

first line of defense: physical and mechanical barriers to pathogen invasi9o

prevent microbes from gaining access to deeper, warmer and moister areas of the body where mesophillic microbes grow and reproduce. ex: skin, mucous membranes

first line of defense: cellular and chemical factors

skins pH, acidity of the stomach and other chemical compounds produced by the body aid in killing or limiting the growth of microbes

first line of defense: microbial antagonsim by our microflora

bacteria on our skin or in our GI tract outcompete most harmful bacteria

what do changes to the first line of defense result in?

increased risk of infection

very few pathogens are able to penetrate _______ __or__ ________:

intact skin or mucous membranes- even a few layers of epithelium can become impenetrable

role of keratin in immunity

makes the skin mechanically tough and resistant to degredation by bacterial enzymes.

how can a pathogen gain entrance to the body through the skin and mucous membranes?

only when skin is ruptured via an abrasi9on, burn or injection

how do mucous membranes provide protection?

provide a non-specific barrier against potential pathogens; consist of a layer of epithelial cells bound by tight junctions, which secrete mucous- this prevents microbial adhesins from binding, and traps the microbes to allow it to be expelled

mucociliary escalator:

our respiratory tract contains ciliated epithelial cells + mucuous- the cilia propels microbes trapped in the mucous out of the lungs and towards the throat where it is swallowed or expelled.

what can make someone more proneto respiratory infections?

damage to the ciliated epithelium due to smoking, pollutants or viral infection

first line defense: cellular and chemical factors

1. skin dryness, temperature and pH
2. perspiration
3. sebaceous gland fatty acid production
4. lysozyme, lactoferrin and lactoperioxidase production in tears
5. digestive acids and bile


1. peristalsis and urination

cellular and chemical factors: skin dryness, temperature and pH

* skin temperature (lower than ideal for mesophiles) and low pH (approx 5) is hostile for foreign microbe survival

cellular and chemical factors: perspiration

skin can sweat, which helps flush away microbes which have landed on us but not yet bound to epithelium

cellular and chemical factors: sebaceous gland fatty acid production

fatty acids produced by sebaceous glands are toxic to many microbes, as they can interfere with their own growth and reproduction

cellular and chemical factors: lysozyme, lactoferrin and lactoperoxidase production in tears

* lysozyme is a secreted enzyme, which causes bacteria to lyse
* lactoferrin is a protein that binds iron, iron is required by all pathogens and therefor cannot compete with lactoferrin for free iron
* lactoperoxidase produce superoxide (OH ion) radicals that are toxic to bacteria

cellular and chemical factors: digestive acids and bile

harsh digestive enzymes and the acidity of the stomach helps kill pathogens

* bile is secreted into small intestine in order to digest fats- when it comes into contact with many bacterial cell walls, it changes the chemical composition of the bacterial cell wall and allows the body to kill the organism

cellular and chemical factors: peristalsis and urination

help remove pathogens from the GI and urinary tract, as does the flushing action of tears to remove pathogens from eyes

microbial antagonism

trillions of microbes live in the oral cavity, skin, upper respiratory tract and colon- they feed off excess nutrients and reproduce, so a new microbe landing there will need to outcompete them for space in nutrients

* microflora target newcomers by secreting antimicrobial peptides called bacteriocins

what occurs when there is a decrease in normal microflora?

can lead to overgrowth of pathogens (opportunistic infection)

what affect do antimicrobials have on normal microflora?

antimicrobials can target their indigenous microflora, and a reduction in their numbers can casue opportunistic infections (ex: women getting yeast infections while on antibiotics)

examples of opportunistic infections:

* patients being treated for S. aureus infections like MRSA may develop vancomycin-resistant enterococcus (VRE) infections during hospitalixatin

where do normal microflora colonise?

* mouth
* skin
* GI tract
* upper respiratory tract
* genital tract

which areas of the body are considered sterile?

* CNS sealed areas (D?T blood brain barrier)
* eyes- lysozyme, lactoferrin, lactoperoxidase inhibit colonization by normal microflora
* internal organs- sealed areas bacteria canot enter
* lungs: cleaned out by macrophages and mucociliary escalator
* bladder- urea kills off anything attempting to colonize this area

when do we acquire normal microflora?

first contact and devedlopment occurs once baby exits the mothers womb

* in the womb, amniotic fluid is a sterile environment
* at birth, microbes tner the mouth and nose as baby passes through birth canal
* first breath= upper respiratory tract colonization
* first meal= establishes GI microflora
* skin microflora is transferred from medical staff and parents to baby

babies are susceptible to ________ becoming their normal microflora:

any bacteria

ex: syphillis, chlamydia, etc.

second line of defense:

when a microbe is able to penetrate the first line of defense by breaching skin or mucous membranes, they are then targeted by other cellular and chemical responses in the body

second line defences:

* fever
* production of interferon
* activation of complement system
* infalmmation
* chemotaxis
* phagocytosis via neutrophils and macrophages

role of a fever in host defense:

* stimulates white blood cells to be released and destroy pathogens
* limits free plasma iron which pathogens thirve on to survive and replicate
* inhibit pathogen growth and reproduction

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what triggers a fever during infection?

* binding of pyrogens (ex. LPS endotoxin)
* direct release of signalling compounds (ex. tumor-necrosis factor, interferon)
* the hypothalamus causes vasoconstriction and shivering to increase the core temperature

overall role of fever during infection:

slows down the growth of mesophillic microorganisms, and can even kill some directly, especially fastidious ones

interferon:

a multipurpose signalling compound


1. alerts cells to viral infection
2. induces apoptosis of virus-infected cells
3. induces antiviral defenses in infected and nearby uninfected cells
4. stimulates immune cells to attack virus-infected cells

when is interferon produced?

one a cell is infected, usually with a virus,

crucial effects of interferon:

one released from the cell, it binds to the membranes of surrounding cells and:

* signals the cell to stop protein synthesis
* also signals the cell to begin degrading RNA
* upregulates protein p53 to promote apoptosis
* alerts nearby cells about a virus and to stop RNA and protein synthesis
* stimulates antigen presentation

what are the three types of interferon?

1. alpha interferon
2. beta interferon
3. gamma interferon

complement system:

a group of more than 30 proteins found in blood plasma which work together in a stepwise manner to initiate a cascade of events that assist in destruction of a variety of pathogens

consequences of the activation of the complement system:

* initation of inflammation
* attraction of phagocytes to sites where they are needed in the body
* activation of leukocytes
* increased phagocytosis by phagocytic cells (opsonization)
* lysis of gram negative bacteria

complement system: initation of inflammation

permits the migration of neutrophils and macrophages across the capillary walls

complement system: attraction of phagocytes to sites of infection and activation of leukocytes

neutrophils move towards the site via chemokines released by damaged cells in repsonse to microbial pathogenesis

* must be activated to be useful

complement system: increased pahgocytosis by phagocytic cells by opsonization

one phagocytes arrive to the site, the compliment system proteins opsonize or sugar coat the cells that need to be phagocytized

compliment system: lysis of gram-negative bacteria

complement proteins can assemble into a membrane attack complex (MAC), which allows C9 to form pores in the cell walls of gram negative bacteria, allowing water, ions and other molecules to move freely in and out of the bacteria, eventually leading to lysis and cell death

why is opsonization more important for gram-positive bacterial clearanced?

the MAC cannot penetrate the thick layer of peptidoglycan on gram-positive bacteria

how is the complement system triggered?

1. classical pathway
2. the lectin pathway
3. the alternative pathway

complement system: the classical pathway

* depends on the production of antibodies by the specific adaptive immune defense
* a specific antibody must first bind to pathogen to form an antibody-antigen complex, which activates the first protein in the compliment cascade, the c1 complex
* the remaining classical pathway complement proteins are recruited and activated in a cascading sequence

complement system: the lectin pathway

similar to classical pathways, but triggered by the binding of mannose-binding lectin to carbohydrates on the microbial surface

* lectins are produced by liver cells and are commonly upregulated in response to inflammation

complement system: the alternative pathway

initiated by the spontaneous activation of the complement protein c3. the hydrolysis of c3 produces c3a and c3b

* c3b attaches to the surface of these microbes and is not degraded how it usually is by our own cells
* once c3b is attached, it will recruit other complement proteins in a cascade to form the MAC

what is the purpose of inflammation?

1. localize an infection
2. prevent the spread of a pathogen
3. neutralize any toxins being produced at the site


1. aid in repair of the damaged site

what triggers inflammation?

* a cascade of chemical mediators and cellular responses which occur when cells are damaged and stressed, or when pathogens susccessfully breach the physical barriers of the innate immune system
* may be triggered by toll-like receptors on the membranes of epithelial cells, leukocytes and others
* toll-like receptors recognize PAMPS (pathogen associated molecular patterns)

process of inflammation:

following an injury, vasoconstriction of blood vessels occurs to minimize blood loss. vasoconstriction followed by vasodilation and increased vascular permeability as a result of histamine release from mast cells= influx of phagocytes to the site of injury/infection

what do toll-like receptors detect ? what is the result?

PAMPS expressed by the microbe

* specific attributes like flagella, an LPS

result= instruct the cells to release inflammatory cytokines to recruit more phagocytic cells to the area to battle a bacterial infection

what else can toll-like receptors detect?

viral patterns, including DsRNA, and binding of DsRNA to the TLR causes the cells to begin secreting interferon to prevent the reproduction and spread of the invading virus

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what is the difference between a TLR detecting a bacteria or fungi versus a virus?

if the microbe is a virus, its genome is detected via TLR3 which causes the cells to secrete interferon to halt RNA and protein synthesis in neighbouring cells to prevent spread, and trigger apoptosis in infected cells

chemotaxis:

the mechanism used in order for leukocytes to locate an infection at a distant site in the body

* damaged or infected cells release chemical signals such as cytokines and compliment protein c5a, which diffuses away from the injury
* many of these cytokines bind to cells of small capillaries and upregulate the expression of various cellular adhesion molecules and receptors
* leukocytes in the blood will stick slightly to the adhesion molecules, slowing down and rolling along the walls near the infected area
* when they reach a cellular junction, they will bind more adhesion molecules, flattening out and squeezing through the cellular junction

what does “rolling adhesion” permit?

allows leukocytes to exit the blood stream and enter the affected areas, where they can bnegin pahgocytosing the invading pathogens

phagocytosis:

the ingestion of particulate matter by neutrophils and macrophages

* pahgocytes are capable of ingestin bacteria, yeast and particulate matter

process of phagocytosis:

once pathogen recognition and attchment occurs, the pathogen is engulfed in a vesicle and brought into the internal compartment of the phagocyte

* TLR’s on the surface of neutrophils and macrophages can aid by first binding to the pathogens surface
* phagosome
* acidifcation of phagosome
* phagolysosome formation

phagocytosis: what is a phagosome?

to engulf the pathogen, the phagocyte forms a pseudopod that wraps around the pathogen and pinches it off into a membrane vesicle called a phagosome

phagocytosis: acidification of a phagosome:

decrease in pH to between 4-5, provides an antibacterial mechanism

phagocytosis: what is a phagolysosome?

the phagosome containing a pathogen fuses with one or more lysosomes, forming a phagolysosome.

* lysosomal enzymes such as lysozyme, phospholipase and proteases digest the pathogen

phagocytosis: what is a repiratory burst?

phagocytes will increase their uptake and consumption of oxygen, to produce superoxide, hydrogen peroxide, and hydroxyl radicals as they are antibacterial

what else can phagocytes do as defence?

some contain microbicidal defensin proteins (neutrophil granules), which can be released into the area around the cell to degrade microbes externally

why are antibodies made?

they are made against things that the compliment proteins do not bind to

what is the difference between neutrophils and macrophages?

neutrophils

* abundant white blood cells in the body and first responders
* phagocytes + respond to cytokines by releasing cytoplasmic granules with antimicrobial properties to damage or destroy nearby bacteria or fungi= degranulation
* self destruct after phagocytosis
* do NOT present antigens

macrophages
* begin as monocytes, which turn into macrophages or dendritic cells after leaving the blood stream
* eat bacteria, viruses, cell debris
* DO present antigens- breakdown foreing microbes and present them to naive B and T cells for antibodies and cytotoxic T cell production

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what is the difference between macrophages and dendritic cells?

both present antigens, dendritic cells do not phagocytize

what if the first and second line defenses fail to clear the microbe?

ex: a virus spreading faster than the localized interferon release can prevent

* duty of the adaptive immune response to TARGET the microbe and kill it off

humoral defense system

humoral= in the lymph/blood

primary mission: development of antigen-specific memory antibodies which provide protecting against current and future infections

how is the humoral defense system built?

macrophages present foreign antigens to B cells and instruct them to begin generating antibodies against the presented antigens

* the antibodies will circulate in the blood, lymph, saliva, etc
* assist in fighting off the infection

how do antibodies fight infection?

varies; involves activation of the complement system (especially in gram negative bacteria), and recruitment of macrophages, natural killer cells, and effector T cells to target the foreign microbe

antigens:

molecules that stimulate the immune system to produce antibodies

antibodies:

proteins produced by the immune system in response to antigens

what triggers the humoral immune system?

pathogen-derived molueculed called antigens (similar to PAMPS, except specific to that microbe)

what can cells use to create antigens?

variety of structures depending on that microbe

* ex: bacterial cells may be associated with capsules, cell walls, fimbriae, flagella or pili
* may be associated with exotoxins and exoenzymes
* viruses posses antigens associated with their capsids, envelopes and adhesins

can an antibody bind to more than one target?

yes; cross-reactivity

* is the origin of several autoimmune diseases such as acute rheumatic fever

most antibodies are very ______ to a particular antigen:

specific; in some cases, an antibody will bind to an antigen that is similar but not identical

= cross reactivity

antibody structure:

glycoproteins created by mature B cells, and used to bind to antigens in the blood and tissue fluids (ex. respiratory and GI tracts, CSF, the wombs, etc.)

antibody structure: Fab and Fc

Fab = antigen binding site; binds to the foreign molecule

Fc= heavy chain; binds to receptors on our immune cells

role of the Fc region of an antibody:

tells us the antibody-dependent complement pathway to activate, and tells our neutrophils and macrophages to target and phagocytose what the antibody is bound to

what are the 5 types of antibodies?

* IgA
* IgD
* IgE
* IgG
* IgM

IgA antibodies:

* >10% of total serum antibody
* secretory antibody, most common and abundant
* found in the mucus secretions that protect the mucous membrane
* also found in breast milk, tears and saliva
* dimer composed of two monomers joined together
* since it is secretory and has 4 fab sites, most important function is to trap pathogens in mucus to be eliminated later

IgD antibody:

* membrane bound monomer found on surface of B cells
* serves as an antigen binding receptor
* not secreted by B cells
* only trace amounts in serum

IgE antibody:

* least abundant antibody in serum
* secreted as a monomer
* role in adaptive immunity restricted to anti-parasitic defenses
* Fc regions binds to basophils and mast cells= release of pro-inflammatory mediators, aiding in defense against parasites

IgG antibody:

* most abundant Ab in human blood
* 80% of patients total antibody production
* small size; able to enter tissue spaces
* only Ab that is able to cross placental barrier= passive immunity to developing fetus

IgM antibody:

* 10% of total antibody production and assembles into a pentamer with 5 monomers of IgM bound together
* first antibody produced and secreted by B cells during primary and secondary immune responses

= pathogen-specific IgM a diagnostic marker during active or recentinfections
* restricted to blood due to size; cannot enter tissues or be secreted
* because all 5 Fc regions bound together, they cannot bind and recruit neutrophils and macrophages
* 10 fab regions allows bidning og 10 antigens at a time
* slow speed of infection by clumping

what are the functions of an antibody?

* neutralization
* opsonization for phagocytosis
* agglutination

neutralization by antibodies:

* binding of an antibody to the antigens of viral or bacterial adhesins. if the adhesins are occluded by antibodies, they cannot bind to our cell receptors

= preventing a virus gaining entry to a cell or a bacteria from adhering

* can also neutralize toxins such as diptheria toxin, by binding it and preventing the toxin from binding to the cell and disrupting protein synthesis

opsonization by antibodies:

* binding of antibodies to microbes in order to signal neutrophils and macrophages to consume them
* fab region binds to the antigen
* fc region binds to fc receptors on phagocytes to signal it to consume the microbe
* useful for bacteria, which attempt to hide from the immune system using LPS or capsule production

agglutination by antibodies:

* when an antibody cross-links more than one microbe and adheres them together
* can prevent large amounts of microbes from binding to cells and prevent dissemination throughout the body
* aggregates easier for kidnsyes and spleen to filter from the blood, easier for phagocytes to ingest
* because IgM and IgA antibodies do not trigger opsonization due to occluded Fc regions, this is the primary way these antibodies protect a patient

T-cell independent B cell activation:

initial form of antibody production- B cells receptors detect repetitive non-protein antigens such as polysaccharide capsules and LLPS

* can cross link multiple B cell receptors on the same cell and initiate a strong enough activation signal


* only IgM produced (which can only neutralize and agglutinate, and are restricted to blood and lymph)
* short lived, remains in blood
* cannot recruit additional immune components
* does NOT produce memory cells

major histocompatibility complex (MHC) cells:

alert the immune system to foreign molecules inside the cell by displaying fragments of antigens on the surface

* found on the surface of all nucleated cells of the body
* two classes: MHC1 and MHC2

MHC1:

molecules are found on all nucleated cells and present normal self-antigens as well as non-self pathogen antigens to the effector t cells incolved in cellular immunity

MHC 2 cells:

molecules only found on macrophages, dendritic cells and B cells and present non-self pathogen antigens for the intial activation of helper t cells

MHC 2 antigen presentation:

* only found on surface of professional antigen presenting cells such as phagocytes and dendritic cells
* after macrophage or dendritic cell recognizes and attaches to a pathogen, it is internalized by phagocytosis
* lysosomes fuse with the phagosome and degradation of the pathogen for antigen processing begins
* antigen bound to immature MHC 2 molecules and are transferred to the surface of the antigen-presenting cell for display

T helper cells:

* activation of T cells occurs after antigen presentation on MH2 molecules
* TH1 and TH2 cells
* after antigen presentation, most of the helper TH1 and TH2 cells become effector T cells and begin to stimulate the various immune cells depending on their type
* relatively short lived
* a subset of TH1 and TH2 cells will become memory cells and will stay in your body for up to 10-15 years

Th1 cells:

upon MH2 antigen presentation, these cells help mature cytotoxic t cells and stimulate phagocytes and NK cells

TH@ cells:

upon MH2 antigen presentation, these cells are critical for maturing B cells into effector B cells, which are capable of producing large amounts of antibodies

* also trigger antibody class switching, allowing B cells to switch between production of IgM, IgG, IgA and IgE antibodies

t-cell dependent b cell activation:

* b-cells interact with antigen at their B cell receptors
* activated B cells recieve an interleukin 4 signal from TH2 cell
* B cell undergoes clonal expansion and differentiates into effector and memory B cells

what is required for antibody production?

activation signals from the antigen binding to the B cell receptors

* depending on how the B cell has detected the antigen and if there is a TH2 cell, the B cell may activate in 1/2 ways:
* t- cell dependent and t- cell independent