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Innate immunity
- nonspecific, responding within 24 hours
- limited diversity responding to core structural components
- usually phagocytose or lyse
- alerts adaptive immunity (dependent on the innate)
- no memory (responds same way each time)
- without innate you would die pretty fast
- physical barriers, bladder, cilia in lungs
- definsins, stomach acid, flora, lysozymes (saliva, sweat, tears, blood)
- complement (circulating and assisting proteins), effector cells
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Innate immunity effector cells
- neutrophils phagocytose kill, increase vascular permeability, secrete cytokines
- monocytes/macrophages phagocytose kill, secrete cytokines, present antigens
- natural killer cells kill virally infected cells, secrete cytokines
- mast cells/basophils release antimicrobial substances and increase vascular permeability
- dendritic cels present antigens, secrete cytokines, transfer antigens to lymph
- responsible for allergic reactions
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Adaptive immunity
- highly specific, responds within 96 hours
- this is what finally rids the infection
- memory
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Adaptive immunity effector cells
- must expand based on pathogen
- T lymphocytes are cell mediated immunity
- they do antigen specific production of cytokines (CD4) to tell macrophage to kill phagoyctosed microbe aka helper T lymphocte
- antigen specific cytotoxic cells (CD8) kill infected cells aka cytotoxic T lymphocyte
- recognize the polypeptide sequence of antigens when presented
- B lymphocytes are humoral immunity
- they do antigen specific production of antibodies and antigen presenting cells
- recognize the 3D structure of antigens
- differentiate into plasma cells
- antibodies cannot cross plasma membrane of cell (only humoral)
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Stem cell origin of immune cells
- common hematopoietic stem cell can become lymphoid or myeloid
- common lymphoid progenitor becomes B, T or natural killer cell
- common myeloid progeniror becomes granuolcyte/macrophage progenetor or megakaryocyte/erythrocyte progenator
- megakryocyte becomes platelets
- granulocyte/macrophage progenitor becomes all white blood cells except for lymphocytes
- then go from naive to effector cells by maturation
- or those that don’t differentiate can become memory cells
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Naive cells
- B cells have low Ig affinity
- only membrane associated IgM and IgD
- T cells when naive just go to lymph nodes
- once activated effector, T cells change surface molecules to leave lymph node
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Innate immune system receptors
- recognize broad classes aka PAMPs, unique to bacteria invariant structures that are immunostimulatory
- coded in germline instead of through genetic recombination
- nonclonal
- descriminate self
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Patter recognition receptors (PRRs)
- surface, endosomal, cytoplasmic
- toll like receptors (TLRs) can be surface or endosomal and recognize cell wall, flagellin, fungal mannans, lipoprotein etc
- PAMPs endosomally can be ssRNA
- activate NFkappaB or IRF-3 transcription factors cause more cytokines, chemokines, selectin which do inflammation, vascular permeability, activation of immune response
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Cytokines
- some allow T and B to talk, some activate CD4 T and CD8 T lymphocytes
- lymphokines (interleukin is subset), monokines, chemokines (B and T lymphocyte talk), interferons
- low molecular weight
- transient
- paracrine or autocrine
- extremely potent
- act in network, multiple can have same effect and different receptors can recognize same cytokine (redundant and pleiotropic)
- synergistic, additive, antagonistic
- few like IL-1 endocrine to hypothalmus in brain, yay fever!
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Epithelium innate response
- physical barrier
- local peptide antibiotics
- intraepithelial lymphocytes kill
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Mononuclear phagocytes
- monocytes differentiate to macrophages upon leaving
- then to microglial in CNS
- to kupffer in liver
- to alveolar macrophages
- to osteoclasts in bone
- phagocytose and destroy by release of granule contents
- recruited to infection site by microbial signals
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Natural killer cell
- innate response
- kill by recognition of MHC
- or downregulation of MHC
- respond to IL-12 produced by macrophages with phagocytosed microbe respond with interferon gamma (IFN) gamma telling macrophage to produce mediators that kill phagocytosed members (feed forward by further activation)
- Fc receptors on natural killer bind antibodies on infected cell or tumor cell and kill it
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Costimulation by innate
- stimulation of T and B lymphocytes by antigens is necessary but not sufficient for activation, proliferation, differentiation
- additional signals come from innate immune cells are required
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Immunoglobin
- aka antibody
- recognize proteins, polysaccharides, lipids, nucleic acids, small molecules, conformational shapes
- found on B cells, thus BCR
- heavy chain light chain, hinged (IgG)
- variable N termini called complementary region lock and key to epitope
- rest is constant
- divalent
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Epitopes
- regions of an antigen that are recognized by immune system
- can be multiple epitopes on any given macromolecule
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Special immunoglobins
- IgG dominating and in placenta (g for gestation)
- does everything, neutralizes, opsonizes, activates complement, antibody dependent cytotoxicity by natural killer cells, feedback inhibition of B cell activation
- IgM forms pentamers with J chain, first synthesized, activates complement, also predominates plasma and blood stream with IgG
- IgA forms dimers with J chain mucosal immunity (transport across mucosal membrane requires dimerization so that it can bind Poly-Ig receptor to transcytose), breast milk, neutralization
- IgE mast cell degranulation, thus epithelial, helminths
- modify C terminus by polyadenylation alternate splicing, class switching, membrane to secreted if signaled (absence of signal makes the membrane)
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T cell receptor
- basically just snap off a branch from the Y shape of an antibody
- alpha and beta chains (or gamma and delta)
- variable and constant
- requires MHC presentation to bind
- only bind 1-3 aa of polypeptide and MHC
- do not bind non proteins, these are thymus independent antigens
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Genetic recombination
- heavy chain has VJD, light VJ
- beta chain has VJD, alpha VJ
- first somatic recombination D and J, then somatic of V and JD
- V is variable, thus codes most of the variability
- transcribe and splice
- also can remove or add nucleotides at the V-J or J-D junctions for more diversity
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Heavy chain isotype switching
- B cells signaled by T lymphocytes
- absence of signal keeps making IgM
- CD40 ligand, cytokines tell it to make different classes IgG etc
- S genes are after VDJ and are points between which activation induced deaminase occurs
- this deletes the C (and S) genes between these two points
- the next C gene will be used for the stop codon and after splicing out the remaining combined S genes and junk, you get the new isotype protein
- irreversible
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MHC variability
- lots of variability
- reason for tissue rejection
- many different alleles at different sites of variability
- most allelic variation at peptide cleft domain
- gives people different affinities
- codominant and most people are heterozygous, thus 4 possibilities
- polygeny with three different genes
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MHC class 1
- all cells of body except for RBC (even T cells!)
- present antigens to effector cytotoxic T cells
- around 10 amino acids presented
- alpha 1 and 2 are peptide cleft domain
- alpha 3 is transmembrane, beta 2 is just there
- CD8 receptor binds to alpha 3
- cytosolic peptides proteasomed and transported into ER by TAP (transporter associated with antigen processing)
- can work on viral expressed proteins
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MHC class 2
- B cells, macrophages, dendritic cells, thymic epithelium (maybe T cells)
- present antigens to helper T cells
- around 20 amino acids presented
- beta 1 and alpha 1 are peptide cleft domain
- beta 2 and alpha 2 are transmembrane
- CD4 domain binds to beta 2
- peptides degraded in endocytotic vesicle
- MHC class 2 escorted from golgi to endosome by binding Ii (invariant chain)
- binding portion of Ii is CLIP and DM in endosome proteolyses this
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MHC peptide binding features
- broad specificity
- only can bind peptides
- degraded if not binding peptide
- peptide bound for as long as required (days)
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Dendritic cells
- Langerhans in epidermis
- presentation often via MHC class 2 pathway
- when activated express CCR7 (chemokine receptor)
- chemotaxis to T cell zone of lymph node via CCR7
- can do cross presentation where it phagocytoses a virally infected cell, gets antigen into cytosol, and epresses its antigen on MHC class 1 and presents to naive cytotoxic T cells (think cross presentation like it uses endosome but does MHC class 1)
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Thymic selection
- positive selection for T cells that have any affinity, those without die of neglect
- negative selection for T cells that have too strong of affinity, eliminated
- there is a sample of antigens from all tissues at some level of concentration
- problems in this or with macrophages and dendritic cells (required for presentation) can cause problems
- CD4 T cell is the one effected by macrophage and dendritic issue
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T cell receptor associated polypeptides
- alpha associated with CD3 delta and beta asociated with CD3 gamma and each with a CD3 epsilon
- all transmembrane external
- two transmembrane internal zeta chains
- these help transport T cell receptor to the membrane and transduce signals to interior (electrostatic except zeta)
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Bone marrow selection
- check for specificity with self using IgM
- no self reaction migrates it to periphery
- recognizes too well can be modified by trying new downstream J agent or is deleted
- if recognizes a soluble self molecule it is changed to not recognize the molecule as well and it migrates to periphery as anergic B cell
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Antigen receptor mediated signaling in B cells
- binding causes phosphorylation of Ig alpha and Ig beta, which activates cascades that can change transcription by PLC or MAP (Ras G protein)
- uses Myc, NFAT, NFkappaB, AP-1
- causes clonal expansion, increased cytokine receptor expression (better response to cytokines), migration to edge of follicle for T cell interaction, secretion from of IgM
- also upon binding it can stimulate CR2 copmlement
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B cells to T helper cells interaction
- B cell has Ig that can bind epitope
- receptor mediated endocytosis of antigen
- presentation of epitope for T helper cell (not necessarily the same epitope)
- B cell expresses costimulators for T cell activation like B7
- T cell recognizes antigen
- B cell has CD40 that binds to CD40 ligand on T cell when they come together, CD40 ligand is only expressed when the T cell becomes activated by binding the antigen
- in response T cell secretes cytokines to B cell to induce B cell proliferation and differentiation (promotes isotype switching) and becomes plasma cell
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Higher affinity B cells
- via heavy and light chain mutations associated with long time of exposure to antigen
- follicular dendritic cells trap antigens so that they cannot circulate
- high affinity B cells can pull these away to the germinal center
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Termination of B cell activation
- Fc receptors block activating signals from antigen receptor when it binds antigen
- the bound antigen probably has secreted IgG bound as well
- Fc receptor activates a phosphatase to kill the kinase activity and inhibit pathway via immunoreceptor tyrosine-based inhibition motif (ITIM)
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B cell subsets
- Follicular B cell in germinal center specifically do isotype switching, high affinity, and live long plasma cells
- marginal zone B cells in lymph organs recognize polysaccharides, lipids etc, mainly IgM and short lived plasma cells
- B-1 B cells same as marginal but live in mucosal tissues and peritoneal cavity
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Secondary antibody response
- memory B cells set higher antibody baseline
- faster expansion and a higher antibody concentration at peak
- followed by even higher antibody baseline and more memory B cells
- more maturation so higher affinity
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Functions of antibodies
- neutralize microbe and toxin
- opsonization and phagocytosis of microbes
- cytotoxicity via Fc receptors on natural killer cells
- activate complement cascade causing lysis of microbes, phagocytosis of microbes, inflammation
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Opsonization
- Fc receptors on macrophage or neutrophil (or eosinophil)
- bind to antibodies on microbe
- Fc signals phagocytosis
- CR1 can bind complement C3b or C4b to cause phagocytosis
- (this is opsonization)
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Fc receptors
- bind antibodies
- specific to certain antibodies
- example Fc gamma RI and Fc gamma RIIA bind certain IgG
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Complement system basic mechanism
- activation by 3 different pathways
- steps are proteolysis of complement protein with larger chunk remaining bound to complex at surface of microorganism and small peptide diffusing away to do things
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Classical pathway complement activation
- antigen antibody complex binds C1 complex (C1q, C1r, C1s), which were there to begin with, to form a protease
- protease cleaves C2 and C4
- cleaved C2 and C4 larger chunks become C3 convertase
- C3 convertase cleaves C3 to kick off the cascade
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MB lectin pathway complement activation
- mannose binding lectin binds mannose on pathogen
- this causes C4 and C2 cleavage
- cleaved C2 and C4 larger chunks become C3 convertase
- C3 convertase cleaves C3 to kick off the cascade
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Alternative pathway complement activation
- C3 binds to microbe surface
- C3 spontaneously cleaves to C3b and associates with Bb to form C3 convertase
- C3 convertase cleaves C3 to kick off the cascade
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Complement activation pathway
- early steps
- C3 convertase recruits and cleaves C3 to form C3b (binding) and C3a (small and leaves to do chemotaxis)
- C3b and the other two are now a C5 convertase
- C5 convertase recruits and cleaves C5 to form C5b and C5a
- C5a goes off to do inflammation
- C3 steps could also be C4
- late steps
- C5b recruits and associates with C6 C7 and C8
- C7 and C8 are membrane bound so it no longer needs the C5 convertase
- recruits C9 which polymerizes with other C9 to form a MAC pore
- lyses cell
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Functions of complement
- opsonization to enhance phagocytosis
- lysis via MAC (C9)
- C3a C4a C5a can recruit and activate leukocytes
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Regulation of complement
- proteins regulate it
- complement regulator (soluble protein) inhibits the protease
- CR1 (complement regulator) on our cells and CDs on our cells prevent complement from attacking self
- CR1 also DAF displaces Bb from C3b
- CR1 also MCP cleaves C3b to prevent action (I mediated proteolytic cleavage producing iC3b)
- many other ways
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T cell immune synapse
- T helper cell activation
- TCR antigen recognition of peptide and MHC class 2
- PAMPs activate APC to upregulate CD80 and CD86
- CD28 does signal transduction (along with CD3 and zeta chains) and binds upregulated CD80 and CD86 on antigen presenting cell
- adhesion so that multiple antigens can be presented to T cell
- adhesion through LFA-1 (beta2 integrin) on T cell binding ICAM-1 on antigen presenting cell
- LFA-1 made high affinity by chemokines secreted by antigen presenting cell
- cytokines released onto T cell to expand and differentiate it
- expansion by IL-2 autocrine to IL-2R
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Cytotoxic T cell activation
- requires T helper cell activation if interacting with dendritic cell MHC class 1
- T helper cell nearby or on same dendritic cell releasing IL-2 to cause expansion
- low levels of IL-2 are sufficient autocrine for expansion if interacting with MHC class 1 of infected cell (including infected dendritic cell)
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T cell activation gene expression
- First increase c-Fos and c-Myc transcription
- hours later make CD40 ligand and Fas ligand
- then hours later IL-2 and IFN gamma
- followed by IL-2 receptors
- downregualtion of L selectin to get out of lymph node (binds to high endothelial cells)
- upregulation of E and P selectins to get through endothelium in periphery
- downregulation of CCR7 which binds CCL19 and 21 chemokines for integrin activation and chemotaxis
- instead it upregulates other ones like CXCR3 for integrin activation and chemotaxis
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Interleukin 2 receptor regulation
- naive T cell has low interleukin 2 receptor affinity
- IL-2Rbeta and IL-2Rgamma c (common gamma chain), thus 2 chains
- when activated it expresses IL-2R alpha that complexes to make a 3 chain complex
- this has high affinity for IL-2
- this is so that you do not cause spurious expansion of nearby T cells that are not specific to the antigen
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CD40 signalling
- bidirectional
- CD40 ligand expressed when activated
- T helper cells have CD40 ligand (CD40L)
- binds to CD40 signaling T cell to make cytokines
- APC can express CD40 and when engaged with CD40L it tells APC to make more MHC
- B cells can also express CD40 and they receive the cytokines that it produces (just mentioned) telling them to differentiate to plasma cells, isotype switch and secrete antibodies
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Different subtypes of T helper cells
- Th1 from IL-12 transcription factor Tbet secretes interferon gamma IFN gamma and TNF alpha, which stimulates IgG switch and acts on macrophages and sort of neutrophils, killing what’s in their lysosomes
- Th2 from IL-4 transcription factor GATA3 secretes IL-4, which stimulates IgG and IgE switch, and IL-5 which activates eosinophil to kill (helminth) and IL-13 which works with IL-4 to activate macrophage to do tissue repair and fibrosis (proinflammatory)
- Th17 from IL-1, IL-6, IL-23 and TFG beta transcription factor RORgamma t secrete IL-17 and granular macropahge colony stimulating factor GM-CSF to act on endothelial cells and increase neutrophil response, also IL-17 induces antimicrobial peptides in epithelial cells
- Tregg from TGF beta transcription factor FoxP3 secretes TGF beta and IL-10 regulatory cytokines
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Delayed type hypersensitivity
- type IV hypersensitivity
- antigen injected
- recognized by memory T cells (Th1) which recruit and activate lymphocytes and macrophages
- 1000s per Th1
- lymphocytes and macrophages cause inflammation (non specific phagocytosis)
- if you inject T cell sera into someone else and try and infect them, the macrophages are the most important part of the sera for seeing a immunizing effect in the other person
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CD8 killing of target cells
- conjugation via MHC class 1 with TCR
- release of granules or granzymes via exocytosis
- enter target cell by perforin molecule (pore)
- perforin made by cascade similar to complement cascade
- granzymes cause apoptosis
- alternate route by FasL on cytotoxic T cell that interacts with Fas on target cell to cause apoptosis
- other ways like granulysin
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Viral evasion of immune system
- herpes simplex virus interferes with TAP to inhibit MHC class 1 presentation
- cytomegalovirus inhibits proteasome to inhibit MHC class 1 presentation
- epstein-barr virus inhibits proteasome too and also make surrogates of IL-10 onto macrophage and dendritic cell to inhibit activation
- pox virus makes surrogates of IL-1 and interferon gamma (INF-1) to block cytokine activation of effector cells
- we use something similar to the pox pathway for rheumatoid arthritis
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Interferon gamma
- IFN gamma
- causes IgG switch
- activates macrophages and neutrophils to a lesser extent
- secreted by natural killer cell and Th1
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