BIOTECH 3IV3

Elie Metchnikoff

a russian scientist who won the 1908 nobel prize for studying immune system

discovery:

• while studying starfish larvae, he noticed moving, amoeba-like cell inside them

experiment:

• poked a starfish larva w a tiny splinter and watched mobile cells swarm around the splinter and consume it

term:

• Phagocytes → cells that eat

conclusion:

• these eating cells protect animals by destroying invading germs

Immune system

a complex network of organs, cells, and signaling molecules work together to protect the body from harmful substances, microbes, and cell changes that make you ill

Subsystems:

1. Innate immunity - “first responders”

   • recognizes: molecular patterns DAMPs (damage-associated) and PAMPs or MAMPs (pathogen- or microbe-associated)

2. adaptive immunity - highly specific, has memory

   • recognizes: antigens

Innate Immunity

Physical barriers → initial obstacles microbes must overcome to invade the host

1. skin

   • dermis → tightly woven fibrous connective tissue

   • epidermis → many layers of epithelial cells

     • outermost = dead, filled with keratin

       • repels water, maintain dry envr

2. mucous membrane

   • digestive, respiratory, genitourinary tracts

   • constantly bathed in secretions - always wet

   • peristalsis of intestines and mucociliary escalator of respiratory tract remove microbes

3. antimicrobial substances

   • protect skin and mucous membranes

   • salt accumulates from pespiration

   • lysozyme degrades peptidoglycan

   • peroxidase enzymes break down hydrogen peroxide

   • lactoferrin binds iron

   • defensins form pores in microbial membrane

4. normal microbiota (flora)

   • production of toxic cmpds

     • propionibacterium degrades lipids, makes fatty acid (GI tract)

     • e. coli synthesize colicins in GI tract

     • lactobacillus in vagina produce low pH

   • disruption here. = predispose person to infection

Hematopoiesis

the process of making and developing blood cells

Origin: all blood cells come from hemtopoietic stem cells (HSCs) in the bone marrow

Development: signals called colony-stimulating factors (CSFs) tell stem cells to develop into diff blood cell types

Location and movement:

• blood cells travel through the circulatory system

• always present in normal blood, some live in body tissues

• immune cell # increases during infections

Categories:

1. red blood cells (erythrocytes) - carry oxygen

2. platelets (megakaryocytes) - help with blood clotting

3. white blood cells (leukocytes) - function as immune cells for defence

Leukocytes

immune cells for defence

1. granulocytes

   • contain cytoplasmic granules

     1. neutrophils → engulf and destroy bacteria

     2. basophils → allergic rxns, inflammations

     3. eosinophils → fight parasitic worms, allergic rxns

2. mononuclear phagocytes

   • consist of mononuclear phagocyte system (MPS)

   • includes monocytes and cell types that develop from them as they leave blood stream

   • macrophages → sentinel cell, diiferentiate from monocytes

   • differentiate from monocytes

3. dendritic cells

   • sentinel cells, “scouts”

   • engulf materials in tissues, bring to cells of adaptive immune system

   • develop from monocytes, also others

4. lymphocytes

   • responsible for adaptive immunity

   • B cells, T cells → highly specific in antigen recognition

   • Natural killer (NK) cells → lack specificity

Cell communication

cells send and receive signals to coordinate their responses

1. surface receptors

   • “eyes and ears”

   • span the cell membrane

   • a specific ligand binds to the receptor → triggers a cell response

2. adhesion molecules

   • help cells stick to other cells

   • ex. endothelial cells stick to phagocytic cells → allow to exit bloodstream

3. cytokines

   • “voices”

   • small signalling proteins released by cells

   • travel to other cells and bind cytokine receptors

   • can cause:

     • growth

     • differentiation

     • movement

     • cell death

   • work at very low conc

   • effects can be:

     • local → nearby cells

     • regional → cells in an area

     • systemic → throughout the body

Types of cytokines

1. chemokines → attract immune cells to a specific location (chemotaxis)

2. colony-stimulating factors (CSFs) → increase and help develop white cells

3. interferons (IFNs) → control viral infections, regulation of inflammatory response

4. interleukins (ILs) → produced by leukocytes, important in innate and adaptive immunity

5. tumor necrosis factor (TNF) → inflammation, apoptosis

Molecular patterns detected by Pattern Recognition Receptors (PRRs)

PRRs → receptors that detect signals of infection or cell damage

• recognize common patterns rather than specific microbes

1. PAMPs/MAMPs

   • PAMPs = pathogen-associated

   • MAMPs = microbe-associated

   • commonly found on microbes

   • examples

     • LPS → bacterial cell walls

     • peptidoglycan → bacterial cell walls

     • lipoteichoic acid

     • flagellin

     • viral RNA

2. DAMPs

   • danger-associated

   • released or exposed when our own cells are damaged

Types of Pattern Recognition Receptors (PRRs)

1. toll-like receptors (TLRs)

   • where? membranes

   • detect what? PAMPs

     • plasma mem TLRs → microbes outside the cell

     • endosome/phagosome TLRs → materials taken into the cell

   • detection? sends signals to the nucleus

   • causes gene exp → can trigger inflammation, antiviral responses

2. nod-like receptors

   • where? cytoplasm

   • detect what? MAMPs

   • help activate protective immune response

   • can form inflammasome

     • activate strong inflammatory responses

     • can lead to cell death

3. rig-like receptors (RLRs)

   • where? cytoplasm

   • detect what? viral RNA (PAMPs/MAMPs)

     • dsRNA

     • RNA with unusual 5’ triphosphate end

   • detection? production of interferons

Interferons

cause cells to express inactive antiviral proteins (iAVPs)

• protein kinase R, RNase L)

iAVPs are activated by dsRNA (becomes AVP) to degrade mRNA

• stop protein synthesis, undergo apoptosis

The Complement System

• “complements” the adaptive immune response

• made of proteins in the blood and tissues

  • named C1-C9

  • can split into fragments

    • ex. C3 → C3a + C3b (split by C3 convertase)

• all 3 pws eventually make C3 convertase

Pathways

1. Alternative pw

   • triggered by C3b attaching to a foreign cell

2. Lectin pw

   • triggered by mannose-binding lectins (MBL) binding on mannose on microbial celsl

3. Classical pw

   • triggered by antibodies bound to antigen

Complement system - 3 main outcomes

1. opsonization

   • C3b sticks to bacteria/foreign paticles

   • makes them easier for phagocytes to engulf

     • C3b = tag for eating

2. inflammation

   • C5a attracts phagocytes to the area

   • C3a + C5a = increased blood vessel permeability

   • cause mast cells to release inflammatory signals

3. lysis

   • membrane attack complex (MAC) forms holes in foreign cell mems

   • made from C5b, C6, C7, C8, C9

   • can destroy gram-negative bacteria

Complement system - regulation

prevents complement from attacking our own cells

host-cell molecules help inactivate C3b

prevents unnecessary opsonization and complement activation

Phagocytosis

Phagocytes engulf and digest material, pathogens

Process

1. chemotaxis - phagocyte recruited by chemoattractants

   • signals include chemokines, C5a, microbial products

2. recognition and attachment

   • direct — receptor binds mannose

   • indirect — binds to opsonin

3. engulfment

   • phagocyte surrounds the invader w pseudopods → forms phagosome

4. phagosome maturation

   • phagosome fuses with endosomes/lysosomes

   • forms phagolysosome

   • pH = lowers, digestive enzymes added

5. destruction and digestion

   • invader is killed and broken down

     • ROS + nitric oxide damage it

     • enzymes degrade it

     • defensins damage its membrane

     • lactoferrin binds iron needed by microbes

     • pH decreases

6. exocytosis

   • vesicle fuses with cytoplasm, expels remain

Attract → attach → engulf → fuse → destroy → expel

(Angela Always Enjoys Finishing Drinks Endlessly)

Types of phagocytic cells

1. MACROPHAGES

   • eat dead cells, debris, invading microbes

   • live for weeks to months

   • always present in tissues

   • can call in other immune cells for help

   • have TLRs on their surface and in phagosomes → detect invaders

   • release cytokines when they detect danger

   • can become activated macrophages → stronger

   • can interact with T cells → inc power

   • if cannot destroy something

     • fuse into giant cells

     • macrophages + giant cells + T cells = granuloma

       • walls of material/microbes that are difficult to destroy

2. NEUTROPHILS

   • “first responders”

   • arrive quickly at sites of infection/damage

   • important during early inflammation

   • MOST ABUNDANT IMMUNE CELLS

   • very powerful

   • live for 1-2 days in tissues

   • die after using their granules

   • can release DNA + granule content to form NETs (neutrophil extracell traps)

     • trap microbes

     • allow enzymes and antimicrobial proteins to destroy them

Inflammatory Response

Purpose: contain the site of damage, localize the response, eliminate invader, restore tissue function

Causes: swelling, redness, heat, pain, LoF

Factors triggering inflammation:

1. PAMPs, DAMPs

   • trigger release of inflammatory mediators

     • pro-inflammatory cytokines

     • chemicals

Inflammatory Response - Process

1. blood vessels dilate

   • vessels become wider => more and slower blood flow to the area

   • causes redness + heat

2. fluid leaks out of blood vessels

   • fluid with complement proteins + clotting factors enters the tissue

   • cause swelling + pain

   • stop bleeding, prevent microbes from spreading

3. leukocytes recruited

   • endothelial cells increase adhesion molecules

   • help grab and slow down phagocytes

4. phagocytes attach strongly

   • increase adhesion molecules

5. diapedesis

   • phagocytes squeeze bw endothelial cells

   • leave the bloodstream, enter the ssiue

6. pus formation

   • dead neutrophils + tissue debris + fluid accumulation = pus

dilate → leak → slow/grab → attach → squeeze out → follow signals → pus

Types of infammation

1. acute inflammation

   • short term

   • neutrophils and macrophages managed to control infection

2. chronic inflammation

   • if acute inflammation fails

   • long term (years)

   • macrophages + giant cells accumulate

   • granulomas form

Inflammatory response - Effects

1. Damaging effects

   • protects the body, but can also damage healthy tissue

   • phagocytes release enzymes + toxic chems → can damage nearby tissues

   • small/local inflammation → minor damage

   • inflammation in sensitive areas → more serious

     • ie. brain or spinal cord

2. Cell death & inflammation

   • sometimes cells sacrifice themselves to prevent spread of infection

   1. non-immunogenic cell death

      • cell dies without triggering much inflammation

      • ie. apoptosis

      • “quick death”

   2. immunogenic cell death

      • dying cells release DAMPs

      • DAMPs trigger inflammation and immune responses

      • ie. necroptosis, pyroptosis

      • “loud death”

Fever

most common host defense mechanism

strong indicator of infectious disease, esp bacterial

How it happens?

• brain normally keep body temp ~37°C

• during infection, pyrogens tell brain to raise temp

  • endogenous pyrogens → cytokines made by our cells

  • exogenous pyrogens → substances produced by microbes

How does it help?

• slow bacterial growth

• can speed up enzyme activity

• increases effectiveness of immune system

  • inflammation, phagocyte killing, lymphocyte multiplication, neutrophil recruitment, interferon production, antibody production, release leukocytes

Antigens

Antigen → molecule can bind specifically to:

• antibody

• B-cell receptor

• T-cell receptor

Immunogen → an antigen that can actually causes can immune response

Antigen - types

1. T-dependent antigens

   • most antigens are T-dependent

   • B cell needs T-helper (TH) cell help to become activated

2. T-independent antigen

   • can activate B cells without TH cell helo

   • ex. LPS, molecules w repeating subunits

What Makes a Strong Antigen?

• Proteins → usually strong immune responses

• Lipids → usually weak responses

• Small molecules → usually not antigenic

Epitopes

specific parts of an antigen recognized by the immune system

• AKA antigenic determines

• can be short regions of a protein

• can also depend on the 3D shape of the molecule

• one bacterial cell can have many different epitopes

How does the body “see” antigens

1. in the blood stream

2. antigen-presenting cells

3. bound by antibodies

Adapttive immunity

• develops throughout life as you encounter diff antigens

• develops a specific response to each invader

• takes abt 1 wk or more after first exposure

• has memory

  • first exposure → slow/weaker response

  • re-exposure → faster/stronger response

• vaccines use this immune memory

Key features

• highly specific

• must distinguish

  • healthy self

  • dangerous cells/invaders

Adaptive immunity - Strategy

primary response → first time encounter

• takes longer → 10-14 days

• some activated B cells continue dividing

  • others differentiate to form antibody-secreting plasma cells

secondary response → encountered again

• faster, stronger

1. humoral immunity

   • antigens outside cells

   • ex. bacteria, toxins, viruses in blood/tissue fluid

2. cell-mediated immunity (CMI)

   • antigens inside cells

   • virus infecting a cell

Antibodies

• aka immunoglobulins

• Y-shaped proteins

• made of

  • 2 identical heavy chains

  • 2 identical light chains

• chains held together by disulfide bonds

MAIN PARTS

1. Fab region = arms

• binds the antigen

• contains the variable region, which determines specificity

• antigen-binding site fits a specific epitope

• binding is reversible

• uses many non-covalent bonds

1. Fc region = stem

   • contains the constant region

   • helps other immune system components recognize and interact with the antibody

Antibody-antigen binding - protective outcomes

1. neutralization → prevents toxins, viruses from binding

2. opsonization → enhancement of phagocytosis

3. complement system activation

4. immobilization and prevention of adherence → binding to bacterial flagella or pili inteferes

5. aggregation → two arms of antibody bind separate antigens

6. antibody-dependent cellular cytotoxicity → targets cell for destruction by natural killer cells

Antibodies/Immunoglobulin (Ig) Classes

• Five major classes: igM, IgG, IgA, IgD, IgE

• have same basic monomeric structure

• each class has different CONSTANT REGION of heavy chain

• some form multimers of basic monomeric structure

• each class has a distinct functions and properties

IgM

• 5-13% of circulating antibodies

• SECRETED AFTER ANTIGEN IS ENCOUNTERED

• first class produced during primary response

• principal class produced in response to some T-independent antigens

• pentamer

  • five monomeric subunits = 10 antigen-binding sites

  • aggregates very effectively

  • large size => cannot cross bloodstream to tissue => so primary role in bloodstream infections

• most efficient

• production begins at birth

IgG

• 80-85% of total serum Ig

  • also exits vessels to enter tissues

• provides longest term protection

  • half-life = 21 days

• first and most abundant produced during secondary response

• protect via: neutralization, aggregation, opsonization, complement activation, antibody-dependent cellular cytotoxicity

• transported across placenta to fetus’ bloodstream

• maternal IgG protects fetus and newbord

  • degrade gradually over 6 mths period

  • infant begins producing

• IgG found in colostrum (first breastmilk), absorbed by infant’s intestinal tract

Iga

• monomeric form = 10-13% of serum abs

• most is a dimer → secretory IgA (sIgA)

• most abundant Ig class produced

• sIgA → important in mucosal immunity

  • gastrointestinal, genitourinary, respiratory tracts

  • saliva, tears, breastmilk secretion

  • protection primarily via neutralization and interfering with attachments of microbes to host

• produced by plasma cells of mucosa-associated lymphoid tissue (MALT),

• secretory component (polypeptide) added to attach antibody to mucus

• ATTACK BEFORE THEY EVEN GO INSIDE THE TISSUES

IgD

• <1% of serum Ig

• involved in development and maturation of antibody response

  • present on surface of immature B-cells

  • function in blood not clearly defined

IgE

Barely detectable in normal blood

tightly bound via Fc region to basophils and mast cells

• allows these cells to detect, respond to antigens

• example

  • antigen binds to two adjacent IgE carried by mast cells → cell releases histamine and other inflammatory mediators

  • basophils and mast cells also release chemicals when IgE binds to normally harmless foods, dusts, pollens that can cause allergic rxns

B lymphocytes and the antibody response

Primary response

1. 1st encounter w specific pathogen

2. IgM produced first, small amts

3. IgG produced later, large amts

4. slower and weaker → immune system is learning to fight the pathogen

Secondary response

1. 2nd counter w same pathogen

2. faster due to memory T cells

3. IgG produced quickly, much higher amts

4. IgM produced, lower amts

Antigen-antibody interaction detection using labeled antibodies

Labeled antibodies: antibodies attached to a detectable marker (enzymes, fluorescent dyes, radioactive tags)

TYPES

1. direct test

   • detects unknown antigen (or bacteria)

   • antigen attached to solid surface

   • labeled antibodies of known specificity added

   • washing removes unbound, binding identifiees antigen

2. indirect test

   • detects antibodies in patient’s serum

   • known antigen attached to surface, serum added

   • unbound antibodies wash off

   • secondary antibodies (eg anti-human IgG) added

   • washing removes, binding identifies primary antibody

Common techniques using antibodies

1. immunofluorescence/immunohistochemistry

2. western blotting

3. ELISA

4. FACS

5. co-immunoprecipitation

Fluorescent antibody (FA) tests (immunofluorescence)

use fluorescence microscopy to locate labeled antibodies bound to antigen fixed to microscope slide

• antibodies may be bound directly or indirectly

Enzyme-linked immunosorbent assay ELISA

antibodies labeled with enzyme, ie peroxidase

enzyme detected using colorimetric assay

TYPES

1. direct ELISA

   • antigen “captured” by antibodies attached to surface

   • enzyme-labeled antibody added to detect antigens

   • commercially available, including rapid grp A strep tests, home pregnancy kist

2. indirect ELISA

   • detect antibodies in human serum

   • eg. blood routinely screened for antibodies to HIV

   • may yield false positives, more reliable test

3. sandwich elisa

   • detects antigen

   • capture antibody attaches antigen to surface

   • detection antibody binds antigen

   • antigen is “sandwiched” bw 2 antibodies

   • signal = antigen present

Data analysis

• typically performed at multiple dilutions/concs

Western blotting

1. SDS-PAGE separates proteins of antigen by size

2. proteins transferred to a membrane, immobilized

3. solutions of antibodies added

4. unbound antibodies washed off

5. labeled secondary antibodies added, unbound washed off

6. label is detected

Diagnostics

1. HIV + control proteins were separated on the gel

2. etc.

   • dark bands = locations of bound antibodies

   • patient a - strong reactive = lots of HIV antibodies

   • patient b - weak reactive = few HIV antibodies

   • patient c - not reactive = no HIV antibodies

Fluorescence-activated cell sorter FACS

• specialized version of flow cytometer

• counts, separates cells labeled with fluorescent antibodies

  • eg. subsets of T cells - CD4+, CD8+

  • label a mixture of cells w monoclonal antibodies, each w a diff dye

Co-immunoprecipitation (Co-IP)

purpose: identify protein-protein interactions

1. antibody binds the protein of interest (bait)

2. bait + its interacting proteins (partners) are pulled out of the cell lysate

3. agarose resin captures the antibody-protein complex

4. wash → removes unbound/non-specific proteins

5. elute → proteins are released and run on SDS-PAGE

6. mass spectrometry (MS) identifies the interacting proteins

Considerations for using antibodies

1. polyclonal vs monoclonal

   • mono - one specific antibody, made using hybridoma tech

   • poly - mixture of antibodies produced by an organism

2. native vs denatured protein

   • some antibodies recognize proteins in their native shape

   • others recognize proteins after they are denatured

   • ex. western blot = denatured, elisa = native

3. surface vs intracellular

   • is the antigen outside/cell surface or inside the cell

   • can the antibody actually reach and bind the antigen

Anatomy of lymphatic system

collection of tissues and organs

1. fluid/lymph → interstitial fluid entered a lymphatic vessel

2. lymphatic vessel → carry lymph

   • begin at peripheral tissues → connections to veins

   • contains WBCs and antigens from tissues

   • returns to lymph nodes → then empties back into circulatory system

3. lymphoid tissues and organs

   • primary = where lymphocytes form/mature

     • B cells mature in bone marrow

     • T cells migrate to thymus for maturation

   • secondary = where lymphocytes are activated and cloned, to contact antigens

     • lymph nodes, spleen, tonsils, adenoids, appendix

     • peyer’s patch → allow sampling of intestinal contents via specialized M cells, part of MALT

     • SALT - skin associated lymphoid tissue

4. lymphocytes → primary cells

   • allow body to resist or overcome infection and disease

Humoral Immunity

• antibody driven

• B lymphocytes or B cells = responsible

  • some form memory B cells → respond quickly if antigen is encountered again

• develop in:

  • bone marrow in humans

  • bursa in birds

• triggered to proliferate, differentiate into plasma cells in response to extracell antigens

  • produce antibodies

    • bind to antigens w high degree of specificity

    • have two functional regions

      • ARMS = bind specific antigen

        • amino acid at the end varies among abs = specificity

      • STEM = “flag” to tag antigen

        • recruit other immune system components to eliminate

    • protect directly and indirectly

      • directly = bind to antigen, prevent attachment to cell

      • indirectly = tag antigen for elimination

B-cell receptor (BCR)

membrane-bound version of B cell’s specific antibody

binding antigen triggers response

• needs helper T cell, sometimes

Cell-mediated immunity

phagocytes activated → cytotoxic T cells activated

T lymphocytes/T cells → mature in thymus

1. cytotoxic T cells

   • destroy abnormal and infected cells

2. helper T cells

   • do not kill pathogens directly

   • “command centre”

3. regulatory T cells

   • suppress response of other T cells and B cells

   • to prevent immune system from mounting a response against self molecules

Helper T cells and Cytotoxic T cells

must be activated to multiply

• confirms antigen signifies danger

1. dendritic cells activate T cells

2. T cell proliferates, differentiate into

   • effector helper T cells → orchestrate humoral and cell-mediated immunity

     • activate B cells, macrophages

     • produce cytokines to direct and support T cells

   • effector cytotoxic T cells → respond to intracell antigens, apoptosis

   • both can form memory cells

Lymphocyte receptors

BCR → specific antibody B cell is programmed to make

TCR → does not recognize free antigen, must be presented by body’s own cell

Clonal selection theory

each B cell or T cell is specific for one antigen/epitope

when that specific antigen appears, matching lymphocyte is selected and make many copies

Process

1. each lymphocyte has a unique BCR or TCR, each recognizes specific epitope

2. antigen enters

   • only lymphocyte w receptors that match it can recognize it

   • other lymphocytes stay inactive

3. additional signals needed to confirm the response is needed, and to prevent self-attack

4. clonal expansion

   • selected lymphocyte multiplies = many identical copies

5. clones become effector and memory cells

   • effector = actively fight pathogen

   • memory = remain long term, respond quickly when same antigen is encountered again

6. if antigen disappears = many responding cells undergo apoptosis

Types of lymphocytes

1. immature → antigen-specific receptor not fully developed

2. naive → have receptors, have not encountered appropriate antigen

3. activated → bound antigen, received confirmation, able to proliferate

4. effector → descendants of activated lymphocytes: plasma, Tc, Th

5. memory → long-lived descendants of activated lymphocytes, secondary response if antigen encountered again

B cell activation by T-dependent antigens

1. BCR binds to antigen

2. antigen is internalized via endocytosis

3. antigen gets degraded into peptide fragments

4. fragments delivered to MHC class II molecules for inspection by Th cells = antigen presentation

   • if t cell receptors binds a fragment = t cells activates B cell with cytokines, initiates clonal expansion

   • no Th cells recognize peptides => B cell may bcome unresponsive to future exposure of antgen → results in tolerance ot antigen

Primary antibody response

some activated B cells become plasma cells

plasma cells

• generate antibodies

• undergo apoptosis after a few days

as long as antigen is present, activated B cells continue to proliferate and differentiate

→ causing antibody lvls (titer) to increase

Affinity maturation

random mutations occur as B cells multiply

form of natural selection

• B cells that bind antigen more strongly = likely to survive and proliferate

Class switching

TH cells can induce some activated B cells to switch

1. B cells → plasma cells (normally secrete IgM)

2. B cells (in lymph nodes) → IgG

3. B cells (in MALT) → IgA

Secondary antibody response

faster and stronger than primary response

• pathogens eliminated before they can even cause harm

• vaccination use this

memory B cells

• greater numbers present

• receptors fine-tuned through affinity maturation

• when activated, quickly become plasma cells, produce antibodies

  • IgG or IA → due to class switching

• proliferating B cells undergo affinity maturation =

  • higher-affinity antibodies after repeated exposure

T-independent antigens

• can activate B cells without helper T cells

• relatively few of them, but medically important

How they work

• they are molecules with many identical, evenly spaced epitopes (eg bacterial polysaccharide capsules)

• bind B-cell receptors at once

• activate B cells

Note

• not very immunogenic in young children

• example: LPS

T lymphocytes

• never produce antibodies

• effector T cells interact directly with target cells

T-cell receptors (TCRs)

each T cell has many copies of its TCR

structure

• two polypeptide chains

  • alpha + beta or gamma + delta

  • each has a variable and constant region

• it looks like one arm of B-cell receptor

does not interact with free antigen

• only antigen presented by another cell

• held in major histo-compatibility complex (MHC) molecules on cell surface

MHC molecules

Types

1. MHC class I

   • carries and presents endogenous antigens

   • produced by all nucleated cells

2. MHC class II

   • arries and presents exogenous antigens

   • produced by antigen-presenting cells (dendritic, B, macrophages)

Diversity in MHC genes

ensures proper presentation and different orientations of proteins from evolving pathogen populations

inherit one set from mother, one set from father

each set has 3 genes:

• HLA-B — at least 1400 diff alleles

• HLA-C — at least 620 diff alleles

• HLA-A — at least 890 diff alleles

Why this matters

• More MHC variants = cells can present a wider range of peptides to T cells.

• Across a population, a pathogen is unlikely to evade everyone’s MHC.

T-cell Activation

1. dendritic cells capture antigens

   • via phagocytosis and pinocytosis

2. detect pathogens

   • toll-like receptors (TLRs) recognize pathogens

   • trigger increased antigen uptake

3. travel and mature

   • dendritic cells migrate to 2ndary lymphoid organs and matures

4. present antigens

   • display antigens on both MHC I and II molecules

5. co-stimulation

   • produce co-stimulatory molecues that signal danger and help activate T cells

   • macrophages and B cells can also present antigens and provide co-stimulatory signals

NOTE

Right side: “self” peptides or harmless material

• self peptides presented by dendritic cells

  • does not produce co-stimulatory molecules.

• Naive T cells that recognize the antigen without co-stimulation become anergic.

• Result: anergic T cells cannot respond and eventually undergo apoptosis.

Tc (CD8) cells — Effector functions

Tc cells check the internal proteins shown on MHC I

See something abnormal → kill the cell by apoptosis

TH (CD4) cells — Effector functions

recognize antigen presented on MHC class II from antigen-presenting cells (APCs)

release cytokines to activate other immune cells

B cells and helper T cells can recognize diff epitopes from same pathogen

Conjugate vaccines

trigger a T-dependent immune response

• attach a protein to a polysaccharide capsule

• B cells recognize the polysaccharide

• helper T cells recognize peptides from the protein

• helper T cells activate B cells → stronger immune response

Haptens

a small molecule that can bind an antibody or B-cell receptor

usually cannot trigger an immune response alone

must bind to a carrier protein to trigger an immune response

example: penicillin can act as a hapten, cause allergic rxns

TH cells in macrophage activation

recognize peptides presented on MHC class II by macrophages

release cytokines that activate macrophages

activate macrophages increase in size, metabolism, lysosome number

• may fuse to form multinucleated giant cells

produce nitric oxide and other toxic cmpds to kill pathogens

Lymphocyte development

both B cells and T cells develop from hetopoietic stem cells (HSCs)

• B cells mature in bone marrow

• T cells mature in thymus

during development, they acquire the ability to recognize specific antigen epitopes

mature lymphocytes can participate in adaptive immune response

Gene rearrangements

creates many diff antigen receptors in B and T cells

each B cell recognizes specific epitope

• the body can recognize millions of diff epitopes without the need of millions of genes

V (variable), D (diversity), J (joining) gene segments

• rearrange by deleting and joining DNA sections

imprecise joining

• adds or removes nts = extra diversity

in B cells, heavy and light chains form separately

• combining them = more unique antigen-binding sites

Negative selection of B cells

occurs in bone marrow

B cells that strongly recognize self-antigens may undergo apoptosis or receptor editing

removes self-reactive B cells to help prevent autoimmunity

Selection of T cells

occurs in thymus

1. positive selection

   • T cells must recognize the body’s own MHC molecules to survive

2. negative selection

   • T cells that strongly recognize self-peptides presented on MHC = eliminated by apoptosis

over 95% of developing T cells die during thymic selection