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T-cell general description
- 1. No secreted form
- 2. detects antigens from within cells using MHC Class molecules
- 3. Matures in thymus
- 4. No affinity maturation
- 5. CD4 = TH1 and TH2; MHC Class II
- 6. CD8 = cytotoxic; MHC Class I
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HLA-DM
- MHC class II-like molecule
- Catalyzes MHC class II CLIP removal
- Catalyzes MHC class II peptide binding
- Peptide editing
- Does not bind peptide
- Increased by IFN-γ
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HLA-DO
- Does not bind peptides
- Negative regulator of HLA-DM
- Not increased by IFN- γ
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Pathogen evasion of MHC difficult because?
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HLA genes
- Human Leukocyte Antigen genes
- MHC class I and class II genes in humans
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Polymorphism
Within species variation at a gene locus
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Gene conversion
Sequence of one gene replaced by sequence of a different gene
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Point mutations
- Replacement substitutions that change an amino acid
- Silent substitutions change codon but not amino acid
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Sequence motif
Set of anchor residues that allow binding to a given MHC class I or class II allele
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Alloreactive
Allogeneic T-cells that react to non-self MHC molecules
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Superantigens
- Cause excessive production of T-cells
- Bind MHC class II already bound to antigen
- Bind TCR
- Creates systemic shock
- Suppresses adaptive immune response
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Superantigen Examples
- SE - Staphylococcal enterotoxins
- TSST - Toxic Shock Syndrome Toxins
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Four main tasks of the immune system
- Immunological recognition
- Immune effector functions
- Immune regulation
- Immunological memory
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Myeloid lineage cells
- Common myeloid progenitor
- Macrophages
- Granulocytes
- Mast Cells
- Dendritic Cells
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Common myeloid progenitor
- Macrophages
- Granulocytes
- Mast Cells
- Dendritic Cells
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Macrophages
- Innate system phagocyte
- In most tissue
- Mature form of monocytes
- Induce inflammation
- Secrete signal proteins
- General scavengers
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Granulocytes
- Neutrophils - innate system phagocytes
- eosinophils - involved in allergic inflammation reactions
- basophils - involved in allergic inflammation reactions
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Mast Cells
- Differentiate inside tissues
- Protect internal surfaces of body
- respond to parasitic worms
- cause allergic responses
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Dendritic cells
- Antigen presenting cells (APC)
- Phagocytic
- Macropinocytosis
- activate T-cells
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Lymphoid lineage
- Both adaptive and innate immune systems
- Common lymphoid progenitor
- NK
- Lymphocytes
- B-cells
- T-cells
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Common lymphoid progenitor
- in bone marrow
- creates antigen specific lymphocytes and NK cells
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NK cells
- recognize and kill abnormal cells
- innate immunity
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Lymphocytes
- Naive lymphocytes
- effector lymphocytes differentiate into B- and T- cells
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B-cells
- Mature in bone marrow
- enter bloodstream as naive mature B-cells
- BCR
- proliferate and differentiate into plasma cells
- produce antibodies and immunoglobulins
- can make memory cells
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T-cells
- TCR
- activated by antigen
- proliferate/differentiate into effector T-lymphocytes
- functions = cytotoxic, helper, regulatory
- can make memory cells
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Central lympoid organs
- bone marrow
- thymus
- generate lymphocytes
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Peripheral lymphoid organs
- lymph nodes
- spleen
- mucosal lymphoid tissues
- maintains mature lymphocytes and initiate adaptive responses
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PALS
- periarteriolar lymphoid sheath
- b-cells along follicles of spleen
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MALT
mucosa-associated lymphoid tissue
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GALT
- Gut-associated lymphoid tissue
- M cells - microfold epithelia of Peyer's Patches in small intestines
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NALT
Nasal-associated lympohoid tissue
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BALT
- Bronchus-associated lymphoid tissue
- overlaid by M cells
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Innate to Adaptive process
- 1. Physical/chemical first defense barriers
- 2. innate response cells
- 3. macrophage secrete cytokines/ chemokines
- 4. cytokines affect nearby cells
- 5. chemokines attract cells out of blood
- 6. inflammation
- 7. Complement
- 8. inflammation
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Three complement activation pathways
- Classical
- Lectin
- Alternative
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Classical Complement Pathway
- Initiated by binding of C1q to pathogen surface
- C1q binds c-reactive protein
- Binds antigen:antibody complexes
- Natural antibodies can bind C1q and activate
- C3 convertase = C4b2a
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Lectin Pathway of Complement activation
- Initiates by binding carbohydrate binding proteins (CBPs) to carbohydrate arrays on pathogen surface
- Homologous to classical pathway
- Mannose binding Lectin (MBL) acts like C1q
- C3 convertase = C2aC4b
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Alternative Pathway of Complement Activation
- Initiated by binding spontaneously initiated C3 component to surface
- C3 convertase = C3bBb
- Does not depend on pathogen binding
- Tickover
- Acts as amplification loop for all three complement activation pathways
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PRR
- Pattern recognition receptors
- recognize PAMPs
- includes: MBL, MMR, fMLP
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PAMPs
- Pathogen associated molecular patterns
- Include: lipoteichoic acid, lipopolysaccharides, unmethylated DNA
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MBL
- Mannose binding lectin
- free in plasma
- initiates lectin complement pathway
- binds phaogcytes
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MMR
- Macrophage Mannose receptor
- similar to MBL
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TLR
- Toll-like receptors
- 10 TLR genes
- produce protein for PAMPs
- limited repertoire
- cell surface receptors
- intracellular acting on endosome membranes (sense pathogen uptake)
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TLR-4
- Detects common bacterial infections
- on Macrophages
- signal bacterial lypopolysaccharides (LPS)
- Represents Toll pathway
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TLR-2
Signals microbial constituents: lipoteichoic acid (LTS) (Gram +) and Lipoproteins (Gram -)
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TLR-3
- signals DS - RNA
- produces cytokine interferon
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Toll pathway
- Initiates with TLR-4
- CD-14 binds LPS
- TLR-4:MD-2 complex targets binds CD-14:LPS complex
- Causes transcript of factor NFκB
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NOD proteins
- Present in cytosol
- Similar to TLRs
- NOD-1
- NOD-2
- Acts in addition to TLR if both present
- Recognize bacterial cell-wall proteoglycans
- Activates NFκB
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NOD-1
- Binds breakdown of (-) bacteria: iE-DAP
- Activates innate immunity
- encoded in CARD 4 gene
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NOD-2
- Binds muramyl dipeptide of (-) and (+) bacteria
- induces α-defensins
- CARD 15 gene
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NFκB activation
- produces cytokines/chemokines
- cell surface expression of co-stimulatory molecules
- activates CD4 T-cells via MHC class II molecules
- Initiates adaptive immune response
- causes migration of APCs to lymph notes by cytokines (TNF- α)
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Adjuvants
- vehicle used to enhance antigenicity
- LPS co-injected with antigen to initiate co-stimulatory molecules necessary for adaptive memory response
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Cytokines
- Secreted by activated macrophages
- induce responses by autocrine, paracrine, endocrine receptor binding
- hematopoietin family of groth homormones and innate and adaptive interleukins (IL)
- TNF family of adaptive and innate
- includes: IL-6, IL-1, IL-12, TNF- α
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Chemokine Description
- chemoattractant cytokines recruit cells to infection
- produced by phagocytes/dendritic cells
- G-protein coupled receptors
- CXCL8 typical of family
- CC chemokines
- CXC chemokines
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Chemokine Roles
- Change conformation of leukocyte integrins adhesion molecules (extravasation)
- Direct leukocyte by chemokine gradient in extracellular matrix (increases toward infection site)
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CC Chemokines
- promote migration of monocytes, lymphocytes, others
- 9 CC receptors (CCR 1-9)
- Example: CCL 2 induces monocyte migration to tissues for maturation to macrophages
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CXC chemokines
- expressed on different cell types
- 6 CXC receptors (CXCR 1-6)
- Example: CXCL 8 promotes migration of neutorphils
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fMLP peptide
- bacterial product
- acts like chemoattractant for neutrophils
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Selectins
- cell adhsion molecule
- leukocyte recruitment
- membrane glycoprotein
- lectin-like domain binds carbohydrates
- binds fucosylated oligosaccharide ligands of leukocytes
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Integrins
- cell adhesion molecule
- binds ICAMs on endothelium to leukocytes
- causes tighter binding
- extravastion integrins: LFA-1, CR3
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ICAMs
- intracellular adhesion molecules on endothelial tissue
- binds to integrins on leukocytes
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Extravasation
Exude from a vessel into tissues
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List the 4 steps of extravasation
- 1. Selectins
- 2. ICAMs
- 3. Extravasation
- 4. Leukocyte migration via chemokine gradient
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Extravasation step 1: selectins
- P-selectin appears via cytokines, histamines, LPS produces E-selctin
- recognizes leukocyte moiety allowing reversible vessel wall adhesion
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Extravasation step 2: ICAMs
- leukocyte integrins LFA-1 and CR3
- increases adhesion of neutorphil
- bind proteoglycans to CXCL8 or chemokines causes conformational change
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Extravasation step 3: Extravasation
- leukocyte crosses endothelial wall
- diapedesis
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Extravasation step 4: Leukocyte migration
- matrix associated chemokine gradient
- CXCL8: recruits neutrophils first
- CCL2: recruits monocytes second
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Cytokine Acute Phase response
- stimulated by endogenous pyrogens (TNF-α, IL-1B, IL-6) and exogenous pyrogens (via TLR-4 signaling E2)
- acts on hepatocytes
- induces 2 acute-phase proteins: C-reactive, MBL
- provides host with antibody like proteins with wide range pathogen specificity
- increases circulating neutrophils = leukocytosis
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C-reactive proteins
- cytokine acute phase protein
- binds phosphocholine of LPS
- Opsonizes
- activates classical pathway via C1q binding
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MBL Proteins of acute-phase response
- increased production during acute-phase repsonse
- opsonin for monocytes which do not express MMR
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Interferons
- proteins that interfere with viral replication
- IFN- α
- IFN- β
- IFN- γ (indirectly induced by virus)
- DS-RNA PAMPs recognized via TLR-3
- Activate RIG-1, MDA-5
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STAT proteins
- Janus-family tyrosine kinase induces transcription
- Oligoadenylated synthetase
- PKR kinase
- Mx
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IFN-γ Production
IL-12 + TNF-α stimulate NK cells to produce
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ITAMs
immunoreceptor tyrosine-based activation motifs
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ILLs
- innate-like lymphocytes
- adaptive but act like innate
- no clonal expansion
- antigen receptors have decreased diversity due to few gene rearrangements
- not via MHC molecules - recognize antigens as consequence of infections
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Natural antibodies
- IgM
- no somatic hypermutation
- Low pathogen affinity
- highly cross-reactive
- binds some self molecules
- not a consequence of infection
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Immunoglobulins (Ig)
- antigen recognition molecules of B-cells
- each B-cell produces 1 specific Ig
- Secreted form = antibody
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BCR
- B-cell receptors
- membrane-bound Ig on B-cell surface
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Antibody description
- secreted Ig
- V-region = variable region antigen binds to
- C-region = constant region (inserted into B-cell membrane in membrane form) defines Ig effector function
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TCR
- T-cell receptors
- V- and C-regions
- bind pathogenic peptide fragments associated with MHC class I & II molecules on cell surfaces
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Ig structural description
- 2 x heavy chains (H chains) both the same
- 2 x light chains (L chains) both the same
- H chain defines class
- both H and L chains have V and C regions
- Fab fragments
- Fc fragments
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Ig L chains
- λ
- κ
- *given antibody has one or the other - not both*
- 2 Ig domains
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Ig H chain
- denoted by lowercase Greek letter of each type (α, γ, δ , μ, ε)
- defines Ig class
- four Ig domains
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Fab fragments
- Fragment Antigen Binding
- corresponds to 2 identical arms of antibody: complete light chain, VH, CH1antigen binding fragment of Ig
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Fc Fragment
- Fragment crystallizable
- Ig fragment with no antigen binding activity
- functional differences
- CH2 & CH3
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Fv
- Fragment Variable
- genetically engineered truncated Fab (V of H linked to V of L)
- small size allows easy tissue penetration
- potential tumor therapy
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Hapten
partial antigen that cannot alone elicit antibody response but can combine with anti-hapten molecules to produce antibodies
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Ig Hv loops
- Hv1
- Hv2
- Hv3 - most variable
- * outer loops for antigen binding
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CDRs
- complementarity determining regions
- composed of combination of VH and VL region for specificity
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Combinatorial diversity
generating different combinations of H and L-chains
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Antigenic determinant/ epitope
- Structure recognized by antibody
- conformational/discontinuous epitopes
- continuous/linear epitopes
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TCR structure
- TCRα
- TCR β
- form heterodimers similar to Ig Fab
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MHC class I molecules
- polypeptide chains (α1 and α2) form peptide binding cleft/groove
- α3 + β2-microglobulin resemble Ig-like domain
- Bind preferentially to CD8 T-cells
- displays peptides from intervesicular pathogens
- binds peptide at endoplasmic reticulum
- on most cells except non-nucleated cells
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MHC class II molecules
- α1 +β1 = binding cleft
- preferentially binds CD4 T-cells
- displays peptide fragments from cystolic pathogens
- normally on b-cells, dendritic cells, macrophages
- stimulate b-cells, macrophages
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RSS
- Recombination Signal Sequences
- noncoding DNA adjacent to points of recombination that guide Ig gene rearrangement
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12/23 Rule
segment flanked by RSS with 12 bp spacer joins with one flanked by RSS with 23 bp spacer
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Ig repertoire diversity main processes
- 1. combinatorial diversity
- 2. Junctional diversity
- 3. Different H- & L-chain combinations
- 4. Somatic hypermutation
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P-nucleotides
palindromic sequences added to ends of gene segments
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N-nucleotides
- non-template encoded
- added by TdT at SS-ends of coding DNA following hairpin cleavage
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Fc region 3 main effector functions
- 1. Recognized by Fc receptors on immune effector cells
- 2. binds complement and initiates compliment cascade
- 3. delivers antibodies to places unreachable without using active transport
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Secondary diversification of antibody repertoire
- only in activated B-cells
- driven by antigen
- 3 mechanisms
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3 Mechanisms of secondary diversification of antibodies
- 1. Somatic hypermutation
- 2. Gene conversion
- 3. Class switching (recombination)
- all initiated by activation-induced cytidine deaminase (AID)
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Somatic hypermutation
- induced by AID
- introduces point mutations into V region of both chains
- alters antigen affinity
- causes affinity maturation
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Gene conversion
- replaces blocks of gene sequence from V region with blocks from pseudogenes
- induced by AID
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Class switching
- induced by AID
- involves C region only
- replaces C μ with alternate isotype
- also called isotype switching
- irreversible DNA recombination
- stimulated by Tcell cytokines or pahtogenic mitogenic signals
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Affinity maturation
- clones have increased affinity for antigen than original BCR
- selected preferentially for secretion
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R loops
bubbles formed when RNA displaces non-template strand DNA
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antigen processing
generation of peptides from intact antigen via modification of native protein
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antigen presentation
display of peptide at cell surface by MHCs
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ABC
- ATP-binding Cassette
- mediates ATP-dependent transport
- TAP 1
- TAP 2
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immunoevasions
- viral interference of antigen presentation of MHC I molecules
- Targets TAP
- Retains MHCI:Peptide complex in E.R.
- dislocation by catalyzing degradation of newly formed MHC I molecules
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