Defence Flashcards
What is the definition of innate immunity
- Born with it
• Natural / native - Ancient
• Plants, insects and all animals have it - Developed by evolution
- Was in place before infection
- Responds in the same way to repeated infections
What are the functions of the innate immune system
- Reacts to microbes + injured cells
- 1st line of defence
• Initial response - Rapid
• Immediate to maximal response in hours - Prevents, controls and sometimes can even eliminate infections
- Many pathogens are evolved to resist + escape the innate immune system
- Eliminated by adaptive immune system
- Innate Immune System keeps the infection in check = immune activation
What are the components of the Innate immune system
- Cells
(Effector cells)
- Phagocytes
• PMN
• M - NK cells
What are the components of the Innate immune system
- Soluble Molecules
- Effector proteins
• Complmenet - Mediators of inflammation
Barriers of the innate immune system :
1 - The epithelial surface
- The epithelial surfaces
- Skin
- Mucosa of gastrointestinal tract
- Mucosa of the respiratory tract
- Prevents the entry of microbes
- Therefore it’s a physical barrier
Epithelial barrier prevent entry of microbes
Mucus = will coat microorganisms
Mucus = prevents adherence to epithelium
Pathogen gets expelled by the movements of cilia
- Pathogen will get expelled - Due to the movements of the cilia
Barriers of the innate immune system :
2 - Chemical Barriers
- Chemical barriers
- Antibacterial enzymes
• Lysozyme - tears and saliva - Antimicrobial peptides
- Defensins and cathelicidins
- Will kill bacteria, by damaging the bacterial cell membrane
- Produced by epithelial cells • PMN ○ Neutrophils • NK cells ○ Natural Killer Cells • CTLs ○ Cytotoxic T lymphocytes
Barriers of the innate immune system :
3 - Chemical Barriers
- Chemical barriers
- Antibacterial enzymes
• Lysozyme - tears and saliva - Antimicrobial peptides
- Defensins and cathelicidins
- Will kill bacteria, by damaging the bacterial cell membrane
- Produced by epithelial cells • PMN ○ Neutrophils • NK cells ○ Natural Killer Cells • CTLs ○ Cytotoxic T lymphocytes
Barriers of the innate immune system :
3 - Chemical (Microbiological) Barriers
- Microbiological barriers • Normal flora non-pathogenic bacteria • Competition - Clinical note - this is an antibiotic treatment - Kills normal flora - Replaced with pathogenic organisms
Innate immune system and disease
- Loss of the integrity will predispose to infection
- Wounds
- Burns
- Genetic defects: cystic fibrosis
- Defective mucus production
- Inhibition of ciliary movements
○ Frequent lung infections
What are the characteristics of the effector cells within the innate immune system
- Cells (phagocytes ) • Myeloid lineage ○ PMN, Mo, Dendritic cells ○ Identify, ingest, destroy - Other cells • Lymphoid lineage ○ NK cells
What are the characteristics of NK Natural Killer Cells
- Kill virus infected cells
- Kill malignantly transformed cells
○ Tumour cells - Express cytotoxic enzyme
○ Lyse target cells
- Kill malignantly transformed cells
Give more characteristics of the Natural Killer Cells
- Kill malignant tumour cells without prior activation
- CD8+ T cells
- Need to be activated and differentiate into CTL
• Cytotoxic T lymphocytes
• To kill target cells
Give more characteristics of the Natural Killer Cells
- Contain perforin
- There are in target cells
Give more characteristics of the Natural Killer Cells
- Contain cytolytic enzymes
- Granzymes A, B
NK cells: how do they recognise the target cells
- The inhibitory receptors
- KIRs • Killer inhibitory receptors - NKG2A • C type lectin receptors - Lecocyte Ig like receptors • LIRs
NK cells: how do they recognise the target cells
- The activating receptors
- NKG2D
- KIRs
- CD16
- Adaptor proteins: which are:
• DAP10
• DAP 12 - Activating receptor recognise infected, or injured cells
NK cells - recognition of target cells
- NK cells have inhibitory and activating receptors
- Outcome of NK cell interaction with other cells is determined by integration of signals from inhibitory and activating receptors
- All healthy autologous nucleated cells have MHC 1
- Inhibitory receptors recognise MHC class
- There is a blockade of signals from activating receptors
- NK cells do not attack healthy autologous cells
NK cells - recognition of target cells
- The virus infected cells
○ These will downregulate MHC 1 - Malignant (tumour) cells
○ Downregulate MHC 1 - Inhibitory receptors are NOT litigated by MHC class 1
- Signals from activating receptors are not blocked
- NK cells attack, and kill the virus infected / tumour cells
Describe the NK cells receptors
- Cytoplasmic tails of inhibitory receptors contain ITIM motif
- ITIM engage molecules phosphatases that block the signalling pathways
• (triggered by activating receptors) - Activating receptors contain ITAM motif
- ITAMs engage in signalling events that promote target cell killing and cytokine secretion by NK cells
- ITAMs are often located not in activating receptors, but in cytosolic portion of adaptor molecule (e.g. DAP 12)
Describe the NK cells killing of target cells : Perforin
- Forms pores
- Delivery of granzymes
Describe the NK cells killing of target cells : Granzymes
- A B C
- Initate apoptosis
NK cells killing of target cells
- Granzymes
- Will activates caspases
- This will lead to apoptosis
- Granzyme B
- Can trigger mitochondrial apoptotic pathway
- Killing of infected cells by NKs
- Eliminates reservoirs of infections
Defects in NK cells and disease
- Human NK cell deficiencies
• As part of broader immuno deeficiencies
○ e.g. Chediak Highashi
• Complete absence of circulating NK cells
• Functional NK cells deficiencies (normal numbers) - Patient have fatal viral infections
• Hepesviruses
Phagocytes characteristics
- Specialised cells • Identify • Ingest • Destroy pathogens • Neutrophils • Mo • Dendritic cells • Belong to the innate immune system
What is the definition of phagocytosis
- Cell eating • Microorganisms • Other cells • Nutrients - Mechanisms of innate immune system
What are the roles of phagocytosis
- Protection from pathogens
- Disposal of damaged and dying apoptotic cells
- Processing and presentation of the antigens (Ag)
- Activation of adaptive immune system
- Links innate and adaptive immunity
- Main phagocytes
- PMN
- Mo
- DCs
Steps of phagocytosis
- Phagocyte mobilisation
- Chemotaxis
- Recognition and attachment
- Engulfment
- Digestion
- Pathogen destruction
Phagocyte defects and disease
- Can be quantitative or qualitative
- Quantitative = fall in number
- Qualitative = fall in function
Examples
1. Chronic granulomatous disease 2. Chediak Higashi syndrome 3. Leucocyte adhesion defects (LADs)
Defects in phagocytosis and disease
- Chronic granulomatous disase
• Mutation in NADPH component
• Defect in oxidation - Phagocytosed microbes cannot be killed
- Get recurrent infections
Describe characteristics of chediak higashi syndrome
- Defect in phagosome lysosome fusion
- Phagocytosed microbes cannot be killed
- Recurrent infections occur
Give the characteristics of Chediak Higashi syndrome
- Rare genetic disease
- Defective gene
• LYSosomal Trafficking Regulatory (LYST)
• Defect in lysosome function - Neutrophils have defective phagocytosis
- Repetitive, severe infections
Leucocyte adhesion defects
- Defect in the beta chain integrins
- Defective neutrophil chemotaxis
Defects in pathogen recognition
- TLRs = recognise pathogens
- Present on:
• Phagocytes
• Mucosal epithelial cells
• Endothelial cells
Defects in pathogen recognition
- cell surface
- TLR1, TLR2, TLR4, TLR5
- Detect extracellular pathogens
Defects in pathogen recognition
- inside cell
- TLR3, TLR7, TLR8, TL9
- Microbial nucleic acids
Defects in TLRs and disease
- Humans lacking TLRs have not been identified
- Polymorphism in TLR genes will presdispose to:
• Bacterial infections
• Asthma
• Autoimmunity (lupus)
Response to infection from the innate immune system
- Rapid response
- Non-specific (generic anti-bacterial, or anti-viral mechanism)
- Most often fails to completely eliminative the infection
What are the characteristics of the innate immune system
- Born with (natural + native immunity)
- In place before the infection gets there - 1st line of defence
- Rapid response (immediate and then is maximal in hours)
- Limited recognition capacity ~ 1000 structures
- Responds in the same way to repeated infections
What are the characteristics of the adaptive immune system
- Triggered by exposure to microbes, so is acquired
- There is a lag time - exposure to maximum response can take days
- Combats pathogens that evade or overwhelm innate system
- More efficient @ eliminative infections
- Exquisitce specificty, > 107 structures
- Has memory + remembers pathogens
- Better / faster with each repeated exposure
What are the components of the adaptive immune system
- T & B lymphocytes
What are the characteristics of humoral immunity
- Humor = fluid
- Serum contains substances that will neutralise the infectious agent - “Antibody Ab”
What is the definition of antibodies
- Antibodies (Ab) = immunoglobulins (Ig)
- Produced in response to foreign structures, (antigens Ag)
- The part of an antigen that is recognised by the antibody is the epitope
- Antibodies have exquisite specificity for their antigens
- Antibodies are very diverse (>107 specificites)
What is the definition + action of antibodies
- They are immunoglobulins
- The migrate in Y globulin fraction of serum
- Fraction of serum that has immune properties
Serum protein electrophoresis
What can antibodies be used for
- As tools for diagnostic assays Ø ELISA Ø Western Blotting Ø Flow Cyotmetry Ø + research
Describe the structure of antibodies
- Tetrameric proteins
- 2X identical heavy chains and 2X identical light chains, which are held together by disulfide bonds
- There are 5 types of heavy chains
• m, g, a, d, e - There are 2 types of light chains
• k, l
Describe the structures of antibodies
- The light chains = 24kDA
- Heavy chain = 50-70kDa
Antibody heavy chain characteristics
- 3-4 constant © domains and 1 variable (V) domain
Antibody light chain characteristics
- 1 constant domain, 1 variable domain
Describe the antibodies structure + function relationship: what are the 2 main roles of antibodies
= Recognition of an infinite number of antigens
- Antigen binding site (Fab)
Describe the antibodies structure + function relationship: what are the 2 main roles of
antibodies
Effector functions of antibodies
- Via Fab
- Bind + neutralise / block entry of antigens
- Other effector functions - mainly mediated by Fc portion
- Interaction with other cells, complement activation
Ø Macrophages
Ø Eosinophils
Ø Both have Fc receptors (FcR)
○ Binding of microbes opsonised by Ab
How to achieve this recognition of infinite antigens
- Antigen binding site = Fab
- VH and VL domains, have 3 hypervariable regions
- These hypervariable regions correspond with protruding loops
- These protruding loops make contact with the antigen
- Hypervariable regions of heavy and light chain, form the antigen binding surface
- Hypervariable regions = complementarity-determining regions
Ø CDR1
Ø CDR2
Ø CDR3
What is special about CDR3
- Has the highest variability
- Due to genetic mechanisms, that ensure Ab diversity
Describe hypervariable regions and Ag binding
- Ig Primary structure = CDRs are separated
- Ig Tertiary (3D) structure= CDRs become adjacent to each other
Describe what happens in hypervariable regions & Ag binding
- Amino acid residues in hypervariable regions, will make contact with the aa residues in Ag
- Biding of diverse antigens by antibodies, is mainly due to hypervariable regions of VH and VL
- The rest of the variable region forms a framework
- This keeps the hypervariable regions in position to interact with Ag
Describe the generation of antibody diversity
- Over 107-109 different types of B cells generated randomly
- Each of these B cells makes different antibody,
- B cells are generated in bone marrow, where they go through different developmental stages
- Naïve B cells populate lymph nodes / spleen. Wait for antigens
Describe the process of clonal selection
- Antigen specific clones of lymphocytes will develop before, and in the absence of the antigens in central (generative) lymphoid organs
- Lymphocyte clones specific for >107 antigens
- Present BEFORE exposure to antigen
- When antigen enters (infection), it activates (selects) the specific lymphocyte clone
• Expansion of antigen specific clone
• Generation of Abs specific for that Ag only
Natural immune responses are polyclonal - describe how
- More than 1 clone of B cells is generated
- More than one Ig is synthesised
- Because there are:
Ø Multiple antigens on organism
Ø Multiple epitopes on each antigen
Ø More than one Ig may recognise the same epitope
How many Ig genes are inherited?
- No complete Ig gene is inherited
- Only bits of genes, gene segments
- Combinations of gene segments allow generation of a high number of different immunoglobulins
Antibodies - generation of diversity:
Describe immunoglobulin gene locus
- Rearrangement of Ig genes during B cell development
- Gene segment recombination = Ab diversity
Antibodies - generation of diversity:
Gene segments for variable & constant domains
- VH encoded in 3 gene segments (V,D,J)
- VL encoded in 2 gene segments (V,J)
- Gene segment recombination = Ab diversity
Describe the heavy chain gene locus - Variable region
Ø 45V (variable)
Ø 23D (diversity)
Ø 6J (joining)
Describe the heavy chain gene locus - Constant region
Ø 9C (different types of heavy chain
Describe the light chain gene locus
- K chain segments
Variable region
Ø 35V (variable)
Ø 5J (joining)
Constant region
Ø 1C
Describe the generation of diversity in antibodies
- There is somatic recombination (V-D-J) joining, addition of N and P nucelotides
- Transcription and RNA processing the 3 cell clones
- Endonuclease cuts randomly after 1 D + before one J and then after 1 V and before DJ
- The free ends are ligated together, to form a functional gene
- This is DNA joining and NOT RNA splicing
What increases the number of antibodies
Ø Junctional diversity and other mechanisms
What is junctional diversity?
- Increases the number of Ab generated
- The enzyme terminal deoxynucelotidyl transferase (TdT)
- Adds random nucleotides (N nucelotides) to the free ends, before joining
• Leads to differences in the sequences of Antibodies that are produced
Ø CDR3 – the most variable portion of the Ig molecule is located at the site of D-J (heavy chain) or V-J joining (light chain)
Ø D-J or V-J joining are the sites of N nucleotide addition => maximum variation of Ig sequence corresponds to CDR3
Why is TdT important
- Generates Ig + TCR gene diversity
- Leukaemia marker
- Useful enzyme in genetic engineering and recombinant DNA work
What is alleic exclusion
- B cells = diploid => 2 alleles of all Ig genes
- Heavy chain
Ø 2 allels
Ø In theory could make 2 different heavy chains - This never happens because of allelic exclusion
What is the mechanism of alleic exclusion (heavy chain)
- As soon as one allele rearranges successfully and heavy chain product is produced
- This will turn off the heavy chain rearrangement
Describe allelic exclusion in light chains
- 2 allels for K chain, and 2 alleles for (upside downY) CHAIN
- In theory they could make 4 different light chains!
- This never happens because of allelic exclusion
What is the mechanism of allelic exclusion (light chain)
- As soon as one allele rearranges successfully and the light chain protein is produced
- Will switch off light chain rearrangement
- Each B cell or B cell clone makes either K or λ chains, never both
- Polyclonal B cells are a mixture of cells making K or λ chains ==> light chain restriction
Describe Ig Expression during B cell maturation?
Ø Functional Ig is expressed as membrane (cell surface) IgM
Ø Membrane IgM acts as B cell receptor (together with IgD)
Ø Antigen recognition by membrane IgM => activation of signalling pathways => B cell activation
Membrane vs. Secreted Abs
Ø B cells have membrane bound Antibody (the antigen receptor)
Ø Upon activation => switch to secreted form of Ab
Ø Generated by differential splicing of exons
Describe the characteristics of Antibodies (Abs)
- There are 2 forms
- Membrane bound on B cell surface => Ag receptor
- Secreted (circulation, tissues, mucosa)
- Membrane bound Igs
=> Ag recognition => B cell activation => Humoral IR
-
With antibodies, what are the characteristics of secreted Igs?
Ø Circulate in the blood
Ø Access various sites to deal with pathogens
Ø They have effector function
1. Neutralisation of microbes + toxins
2. Opsonisation of microbes, to enhance phagocytosis
3. Complement activation (pathogen killing)
How do antibodies work?
Ø There are different classes of Abs. They work best at certain sites
• IgM
• IgG - Blood
• IgA - Mucosa
Ø Different classes of Abs work best against certain pathogens
• IgE - Parasites
Ø Bind to extracellular microbes & toxins
• Neutralise (block adherence and entry)
• Elimination
Ø Opsonisation is the increase of phagocytosis
Ø Lysis
Describe the isotype switching in Antibodies
- During an immune response, the B cells will become capable to produce antibodies of different classes, but without changing specificity
• Respond to the same Ag. - IgM, switch to => IgG, IgA, IgE
- IgG, switch to => IgA, IgE
- Ability to perform different effector functions
- Can therefore deal better with pathogens
- Isotype switch needs signals from the helper T cells
- Isotype switching does NOT alter specificity for the Ag.
- Isotype switching does NOT alter the light chain.
Describe T cell help and the Ab isotype switch
- CD4 0L on the T cell will interact with CD40 on B cells
- Cytokines are produced by the T cell
IFN y => Switch to IgG1, IgG3
IL4 => switcch to IgE
TGF b (& other cytokines) => switch to IgA
Antibodies - isotype switching
- This is the DNA rearrangement process
- Antibodies retain already rearranged variable regions, whilst exchanging constant regions
Describe the Co-expression of IgM and IgD
- IgD is coexpressed, with IgM (works as antigen receptor)
Ø IgM is 1st immunoglobulin to be produced
Ø IgD is produced at the same time with IgM
○ The mechanism for this is differential splicing - Exons for Cμ and Cδ are transcribed as part of a single precursor RNA
- Differential splicing, can remove Cμ exons => now Cδ exons are used => IgD (same VDJ as IgM joined to the Cδ)
What can differential splicing remove
- IgD is coexpressed, with IgM (works as antigen receptor)
Ø IgM is 1st immunoglobulin to be produced
Ø IgD is produced at the same time with IgM
○ The mechanism for this is differential splicing - Differential splicing, can remove Cμ exons => now Cδ exons are used => IgD (same VDJ as IgM joined to the Cδ)
What is affinity maturation?
Ø Antibodies that are produced early during the immune response have lower affinity for antigen
Ø Later on during the immune response, & in secondary immune responses => production of high affinity antibodies
Ø This process will lead to increased affinity of Antibodies = affinity maturation
Ø MECHANISM: Somatic mutation of Immunoglobulin genes are followed by a selection of B cells, that produce antibodies with the highest affinities
• This needs signals from helper T cells
Ø The activated B cells, have a high rate of point mutations targeted by the Ig genes = somatic hypermutation
Ø Somatic hypermutation is localised in rearranged VDJ or VJ (corresponding to the variable region)
Ø Some mutations decrease affinity for binding to Ag
Ø Some mutations increase affinity for binding to Ag
Ø The next step is selection of B cells with high affinity for Ag.
What is the mechanism of affinity maturation?
Ø Antibodies that are produced early during the immune response have lower affinity for antigen
Ø Later on during the immune response, & in secondary immune responses => production of high affinity antibodies
Ø This process will lead to increased affinity of Antibodies = affinity maturation
Ø MECHANISM: Somatic mutation of Immunoglobulin genes are followed by a selection of B cells, that produce antibodies with the highest affinities
• This needs signals from helper T cells
Ø The activated B cells, have a high rate of point mutations targeted by the Ig genes = somatic hypermutation
Ø Somatic hypermutation is localised in rearranged VDJ or VJ (corresponding to the variable region)
Ø Some mutations decrease affinity for binding to Ag
Ø Some mutations increase affinity for binding to Ag
Ø The next step is selection of B cells with high affinity for Ag.
What is the mechanism of affinity maturation?
Ø Somatic mutation of Immunoglobulin genes are followed by a selection of B cells, that produce antibodies with the highest affinities
Ø Affinity maturation needs signals from helper T cells
Characteristics of affinity maturation in B cells
Ø B cells, that have high affinity Ag receptors are selected to survive
Ø B cells with low affinity Ag receptors might fail to survive (competiton for antigen recognition on follilcular dendritic cells, in the germinal centres)
Ø There is preferential selection of B cells, that have high affinity Ag receptors during immune responses
Describe B cell selection in Germinal Centres
- B cell activation by protein antigen & helper T cells
- B cells with somatically mutated Ig V genes and Igs with varying affinities for the antigens
- B cells encounter antigen on follicular dendritic cells (FDC) & present antigen to the TFH cell
- B cells with high affinity antigen receptors will preferentially recognise the antigen on FDCs, interact with TFH cells and are selected to survive
B cell antigen presentation to helper T cells
- There is exit of high affinity antibody secreting memory and B cells
- Somatic mutation and affinity maturation, isotype switching
- B cell proliferation
- Activation of B cells and migration into the germinal centre
Class switching summary
- Occurs by DNA rearrangement
- The arranged VDJ (codes for Ag specificity) is not altered in class switching
Somatic hypermutation
- Ig genes in activated B cells, undergo somatic hypermutation
- B cells which gain higher affinity for antigen are selected to survive
- The result is affinity maturation (increased Ab affinity for Ag)
What is complement?
Ø Means - “to make complete”
Ø Complements / adds to the action of antibodies, e.g. bacterial killing
Ø Originally described as: “Heat sensitive component of serum that could augment the ability of antibodies to inactive antigen”
Definition of complement
- Group of plasma proteins, normally inactive
Ø More than 30 plasma & cell surface proteins
Ø 9 central components of the complement cascade
Ø Complement component 1 (C1) to C9 - Triggers => activation of complement pathways
Ø Part of both innate immunity and humoral immunity
What is the purpose of complement activation
Ø The generation of products that mediate effector functions of complement
What does complement activation involve
Ø Cascade of enzymatic cleavage of complment proteins (proteolytic cascade) [amplification]
Ø Products of complement proteolysis attach covalently to microbial surfaces, or antibodies that are bound to microbes, or other antigens [stabilisation]
Ø Regulatory proteins will inhibit complement activation on the healthy host cells, absent from microbes [specificity]
What is amplification in the complement cascade
- When each activated protease can generate multiple activated proteases in the following step
What is stabilisation in in the complement cascade
- Complement proteins are inactive/transciently active when in the fluid form
- Stable activation when deposited on the microbes
What is specificity in relation to complement
- Complement activation on the microbial surfaces, whilst minimising complement mediated damage to the host cells
What are the roles of complement
- 1. To eliminate microbes
- Pathogen opsonisation - facilitates phagocytosis
- Inflammation - recruitment + activation of leucocytes
- Microbe lysis
MAC: Membrane Attack Complex
(2. Eliminate apoptotic cells + debris
3. Promote clearance of Ag:Ab (immune) complexes
4. Promote B cell activation )
Complement activation pathway: describe the classical pathway
- C1 interacts with antibodies (IgM, IgG), bound to microbes
- Effector mechanism of humoral (adaptive) immunity
Complement activation pathway: describe the alternative pathway
- Direct recognition of microbial structures (innate immunity )
Complement activation pathway: describe the Lectin pathway
- MBL (mannose binding lectin) will recognise terminal mannose residues on microbes (innate immunity)
- Ficolin recognises residues on microbes (innate)
Characteristics of the complement activation pathways
- There are different triggers for initiation of each pathway, but there are similar consequences:
Ø Generation of proteolytic enzymatic complexes
Ø C3 convertase
○ Cleaves C3 in C3a and C3b
Ø C5 convertase
○ Cleaves C5 in C5a and C5b - Proteolytic products of complement proteins identified by lowercase letters afterwards
Ø “a” for smaller product, “b” for larger product - C3 cleavage is crucial for all complement functions
Explain the alternative complement activation pathway
Ø Spontaneous cleavage of C3 in plasma (C3 tickover)
Ø C3 contains a reactive thioester bond (which is hidden)
Ø C3 cleavage induces conformational change in C3b, which exposes this thioester bond
Ø This exposed thioester bond will react with an amino, or hydroxyl group on the surface of microbes to form ester bonds => covalent attachement of the C3b to the surface of microbes/cells
Ø When there is an absence of covalent attachement, C3b remains in fluid phase, rapidly inactivated by hydrolysis => further complement activation is stopped.
Steps of the alternative pathway
- Covalently tethered C3b will bind factor B
- This bound factor B is cleaved by Factor D (plasma protease)
- Generates Bb (large) and Ba (small) fragments
- Bb will remain attached to the C3b
- C3bCb complex, is the alternative pathway C3 convertase
- C3 convertase has main function, to cleave more C3 molecules => amplification of complement activation
- Newly generated C3b deposits of microbial surface
- C3a is a soluble fragment; mediates biological activities
What is the alternative pathway to C3 covertase
- C3bCb complex
Role of C5 convertase in the alternative pathway
- Some C3b molecules will bind to C3 convertase (C3bBbC3b)
- C3bBbC3b is alternative pathway for C5 convertase
- C5 convertase cleaves C5 & initiates late steps of complement activation
Steps of the alternative pathway (late steps )
- C5 convertase will cleave C5 in C5b (large) and C5a (small)
- C5a = soluble fragment, has several biological activities
- C5b remains bound to C5 convertase
- C5b recruits C6 and C7
- C7 = hydrophobic ==> inserts into lipid membranes
- C7 recruits C8
Ø 3 chains, one will insert into the membrane - C5b-C8 complex will recruit C9 ==> C9 polymerises ==> pores
- C5b - C9 is called MAC (Membrane Attack Complex)
- Lysis of microbe by MAC; entry of H20 via C9 pores
Describe the classical pathway
- Initated by binding of C1 to antigen bound IgG or IgM
Ø C1 = multimeric protein complex of C1q, C1r, C1s
Ø C1q binds to antibodies (IgM, IgG1, IgG3)
Ø C1r & C1s = proteases
Ø C1 does not bind to soluble (free) antibody molecules
Ø C1 binds only to antibodies, that are bound to Ag.
Characteristics of C1 complement component
Ø C1q hexamer = 6 globular heads, which are connected via collagen like arms, to a central stalk (“bunch of tulips”)
Ø C1r (x2) & C1a (X2) form a tetramer