Antibodies, Complement, and T Cell Pathogen Recognition Flashcards
What are antibodies also known as?
Immunoglobulins, Ig
What kind of molecules are antibodies?
Proteins
What is the molecular weight of antibodies?
150,000-900,000 kd
What are antibodies derived from?
The immunoglobulin supergene
What do the two ends of the Ig do?
- One end binds to antigens
- The other end is crystallisable, and is responsible for effector functions
What is the end of the Ig that binds to antigens called?
The Fab region
The Fragment of the Molecule that is antigen binding
What is the crystallisable end of the antibody called?
Fc
Are Fc domains constant or variable?
Constant
Are Fab regions constant or variable?
Extremely variable between individual Ig’s
What is true of the progeny of a single clone of B cells?
They are identical
What does the basic, monomeric, Ig molecule have?
- Two light chains
- Two heavy chains
What happens to the heavy and light chains in an Ig molecule?
Intrachain S-S links divide H-L chains into domains that are seperately folded
How many H chain domains does an IgG molecule have?
3
CH1, CH2, and CH3
What is found between CH1 and CH2 in an IgG molecule?
Many cystine and proline residues
What is the region containing the residues between CH1 and CH2 in IgG known as?
The hinge region
What does the hinge region do?
Confers flexibility to the Fab arms
Label this diagram of an IgG antibody
How many classes, or isotypes, of antibodies are there?
5
What are the classes of antibodies categoried by?
Differences exisiting in their H-chain constant regions
How many classes of light chains are there?
2
What are classes of light chains?
- Lambda
- Kappa
What does an individual Ig molecule include?
Referring to light chains
Only L chains (both identical) of one class
Though we all possess Ig molecules, which are made of either
What are the classes of Ig?
- IgM
- IgG
- IgA
- IgE
- IgD
What structure is IgM?
Pentameter
What is the H chain of IgM have?
µ
What is the L chain of IgM have?
λ or κ
What are the subclasses of IgM?
None
What are the functions of IgM?
- Agglutination
- Complement activation
- Primary response
What is the site of IgM?
Blood
What is the structure of IgG?
Monomer
What is the H chain of IgG?
γ
What is the L chain of IgG?
λ or κ
What are the subclasses of IgG?
1-4
What are the functions of IgG?
- Opsonisation
- Complement activation
- Neutralisation
- ADCC (NK cells)
- Transplacental
- Secondary response
What is the site of IgG?
- Blood
- Tissues
What is the structure of IgA?
Monomer or dimer
What is the H chain of IgA?
α
What is the L chain of IgA?
γ or κ
What are the subclasses of IgA?
1-2
What are the functions of IgA?
- Secretory
- Neutralisation of microbes and toxins
What is the site of IgA?
- Mucosa
- Blood
- Breast milk
What is the structure of IgE?
Monomer
What is the H chain of IgE?
ε
What is the L chain of IgE?
γ or κ
What are the subclasses of IgE?
None
What are the functions of IgE?
- Mast cell degranulation
- ADCC (Eosinophils)
What is the site of IgE?
- Muscosa
- Tissues
- Parasites
What is the structure of IgD?
Monomer
What is the H chain of IgD?
δ
What is the L chain of IgD?
λ or κ
What are the subclasses of IgD?
None
What is the function of IgD?
B cell antigen receptor
What is the site of IgD?
B cell membrane
Draw a diagram showing an IgG antibody
- Red - disulphide links*
- Green - light chains*
- Blue - heavy chains*
In what form does IgM exist?
Monomeric or pentameric
Draw a diagram showing an IgM antibody
- Red - disulphide links*
- Green - light chains*
- Blue - heavy chains*
In what form does IgA exist?
Monomeric or dimeric
Draw a diagram showing an IgA antibody
- Red - disulphide links*
- Green - light chains*
- Blue - heavy chains*
What is the linkage between monomers in IgM and IgA?
Via a J chain
Where is IgA the principle Ig?
In secretions, e.g. GI, respiratory, and genital-urinary mucus
How does IgA protect the organism?
By gaining attachment to mucous membranes, but is also found in the blood
What happens as the IgA molecule passes into an epithelial cell from the blood stream?
It receives an additional structure termed the secretory component
What does the secretory component of IgA do?
Transports the IgA through the cell, and remains attached to it at the mucosal surface.
In what ways are antibody specificities determined?
- Combinational diversity
- Junctional diversity
- Somatic hypermutation
What happens in each B cell?
With respect to the gene encoding
The genes that encode for antibodies are recombined from several alternative segments, giving millions of possible combinations
What is the ability of B cell genes to encode for antibodies in different combinations called?
Combinatorial diversity
How many possible combinations are there for the variable region of κ light chains?
Include the number of alternative V and J gene segments
- 40 alternative V gene segments*
- 5 alternative J gene segments*
40 x 5 = 200 variations
How many possible combinations are there for the variable region of λ light chains?
Include the number of alternative V and J gene segments
- 29 alternative V gene segments*
- 4 alternate J gene segments*
29 x 4 = 116 variations
How many possible combinations are there for the variable region of heavy chains?
Include the number of alternative V, D and J gene segments
- 51 alternate V gene segments*
- 27 alternate D gene segments*
- 6 alternate J gene segments*
51 x 27 x 6 = 8262 variations
What is the total number of antibody variations?
Since the variable regions of light and heavy chains contribute independantly to specificity;
(200 + 116) x 8,262 = 2.5 x 106
<em>In practice, there are fewer than this, since not all V gene segments are utilised with equal frequency, and not all variable heavy and light regions will pair successfully </em>
How does junctional diversity arise?
In the third hypervariable region, nucleotides are enzymatically inserted, altering the resultant specificity
What is the third hypervariable region?
- V-J for light chains
- D-J for heavy chains
What happens in somatic hypermutation?
Once B-cells have been stimulated by antigens, they are more likely to undergo a point mutation in the variable regions of rearranged H and L genes. These point mutations result in the appearance of mutant immunoglobulin molecules on the surface of the B cell that are able to interact with antigens
What is the consequence of the mutational event in somatic hypermutation?
The antibody specificity may be altered, conferring greater or less affinity
What happens if a mutational event in an antibody confers greater affinity?
The cell is more likely to be stimulated to divide
What is an increase in affinity due to somatic hypermutation known as?
Affinity maturation
What do secreted antibodies bind to?
Circulating antigen
What is formed when secreted antibodies bind to circulating antigen?
Immune complexes
What do some immune complexes go on to activate?
Complement
What happens to immune complexes after formation?
They are trapped by follicular dendritic cells (FDCs) in the germinal centres of lymph nodes
What allows FDCs to trap immune complexes?
The cells possess receptors for the Fc of IgG and C3b
What may happen to the surface membranes of the FDCs once they have trapped immune complexes?
They may bud off, complete with the attached immune-complex
What is the name of the structures formed when surface membranes of FDCs and immune-complexes bud off?
Iccosomes
What is the purpose of iccosomes?
To present the original antigen to B cells bearing high affinity surface immunoglobulin
What happens to high affinity B cells?
They survive, developing into memory B cells or immunoglobulin secreting plasma cells
What is the ability of high affinity B cells to survive conferred by?
Helper T cells (CD4+) interacting with the B cell-presented peptide
What is the B cell-presented peptide dervied from?
The original antigen
How does the Helper T cell help the memory B cells survive?
The CD40 ligand on the T cell interacts with CD40 on the B cell, making Bcl-xL
What is Bcl-xL?
A pro-survival protein
What happens to low affinity B cells?
They die via apoptosis
What happens to the low affinity B cells that have died via apotosis?
They are phagocytosed by macrophages in the germinal centres
What are macrophages called once they have phagocytosed low affinity B cells?
Tangible body macrophages
What do T lymphocytes play a major role in?
- Defence against intracellular pathogens such as viruses, protozoa, and intracellular bacteria
- Immunity to extracellular pathogens
How do T lymphocytes play a major role in immunity to extracellular pathogens?
By providing help for the antibody response
What do all T cells express?
The CD3 surface receptor
What is the CD3 surface marker specific to?
T lymphocytes
What is the CD3 surface marker often used to do?
Characterise T cells
What do T and B cells both express?
An antigen specific surface receptor (TcR)
How is a huge diversity of TcRs achieved?
A similar process to that of B-cells, rearranging TcR genes for V (variable), D (diversity), K (joining), and C (constant) gene segments
What are individual antigens recognised by?
Clones of T cells
What to the antigen-specific receptors of T cells recognise?
Specific epitopes within the antigen
What are the forms of TcR?
- γ/δ
- α/ß
What % of T cells are of the γ/δ type?
<10%
When do γ/δ T cells become activated?
In infection
How important is the role of γ/δ T cells in the immune response?
They are non-essential in most infections
What are the main functions of γ/δ T cells?
- Cytokine secretion
- Cytotoxicity
What % of T cells are of the α/ß type?
>90%
What do α/ß T cells express?
Either CD4+ (helper) or CD8+ (cytotoxic) surface makers
What do CD4+ helper T cells do?
- Produce cytokines
- Coordinate the immune response
What do CD8+ cells do?
Kill cells that are infected with intracellular pathogens
What intracellular pathogens can CD8+ cells protect against?
- Viruses
- Some bacteria
- Some intracellular protozoa
Give an example of an intracellular bacteria that CD8+ can protect against
Listeria
Give an example of an intracellular protozoa that CD8+ cells can protect against
Malaria parasites
In what form do TcRs recognise antigens?
Short peptides presented in association with ‘self’ Major Histocompatibility Complex (MHC) Class I or II molecules
How many MHC class I genes are there in humans?
3
What are the MHC class I genes in humans?
HLA A, B, and C
How many MHC class II genes are there in humans?
3
What are the MHC class II genes in humans?
HLA DR, DP, and DQ
In general, what do MHC Class I present antigens to?
CD8+ T cells
What are MHC Class I expressed on?
Almost all nucleated cells of the body
What are the MHC Class I antigens bound for presentation derived from?
The cell’s cytosol
Where are the MHC Class I antigens usually synthesised?
Endogenously, within the cell
What kind of antigens are MHC Class I?
Viral
What do MHC Class II present to?
CD4+ T cells
Where are MHC Class II expressed?
On professional APCs
What cells are professionals APCs?
- B cells
- Macrophages
- Dendritic
What are MHC Class II antigens bound for presentation derived from?
Exogenous antigens, such as soluble proteins or extracellular organisms
What are the stages in the class I MHC pathway of processing of protein antigens?
- Antigen uptake
- Antigen processing
- Transport of peptides to ER and assembly
- MHC-peptide transport
- Peptide presentation
What are the stages in the class II MHC pathway of processing of protein antigens?
- Antigen uptake
- Antigen processing
- MHC biosynthesis
- MHC-peptide association
- Peptide presentation
What do HLA represent, in addition to their roles as antigen presenting molecules?
The main antigenic targets in organ graft rejection
When is HLA the main antigenic target in organ graft rejection?
Where these are not matched between organ recipient and donor
What are some HLA molecules associated with?
Increased susceptibility to specific diseases
Give two diseases that are associated with HLA molecules
- HLA-B27
- Ankylosing spondylitis
Why are T lymphocytes so named?
Because of the role that the Thymus plays in their development
Where do pre-T lymphocytes arise from?
Stem cells in the bone marrow
Where do pre-T lymphocytes migrate to?
The thymus
What happens do once they have migrated to the thymus?
They develop into mature, functional T cells
What are mature, functioning T cells capable of doing?
Responding to antigen in the periphery
When are T lymphocytes known as Thymocytes?
During their differentiation and migration through the thymus
What is negative selection designed to do?
Delete T cells whos antigen receptors recognise self antigens with high affinity
Why does negative selection need to delete T cells whose antigen receptors recognise self antigens?
Otherwise they would be autoreactive
What does negative selection achieve?
Self tolerance
What does positive selection ensure?
That cells capable of recognising foreign antigens survive and diffentiate further
Draw a diagram illustrating the process of T cell maturation
How long does the antigen-specific immune response take to develop?
A few days
Why does the antigen specific immune response take a few days to develop?
Due to the need for clonal selection, differentiation, and expansion of antigen-specific effectors
What does the adaptive immune response to infection involve?
Both T and B lymphocytes
What is the integrity of the adaptive immune response based on?
- The ability of B and T lymphocytes to recirculate between the blood and lymphoid tissues
- The ability for activated effort cells to migrate into the somatic tissues at sites of infection
What phases can the adaptive immune response be seperated into?
Where does the induction phase of the adaptive immune response occur?
In the lymphoid tissues
How do the antigens get to the lymphoid tissue during the induction phase?
They either find their own way there, or, more commonly, its transported to lymphoid tissues by migratory Dendritic cells
How can an antigen find its own way to the lymphoid tissues for the induction phase?
- Antigen can pass in the circulation through the spleen
- Pass in circulation in afferent lymph entering lymph nodes
What is the function of dendritic cells?
They are the main type of professional APCs in the induction of the immune response
Where are immature dendritic cells found?
In the periphery
What happens to immature DCs after the capture of an antigen in the periphery?
They migrate to regional lymph nodes
What happens to immature DC’s after they have migrated to regional lymph nodes?
They process the antigen and mature into cells capable of activating naive T cells
What is required to stimulate a naive T cell?
Two signals;
- Recognition of an antigen/MHC complex by the T cell’s TcR
- Interaction of surface molecules on the T cell and the APC
What are the interactions of surface molecules on the T cell and the APC between?
- CD80 and CD86on the APCs
- CD28 on the T cells
What do activated T cells release?
IL-2
What does IL-2 play an essential role in?
T cell proliferation and differentiation
Draw a diagram illustrating how a co-stimulatory signal and specific signal activates a T cell
What expresses the molecules that provide co-stimulatory signals to T cells?
Only professional APCs
What is the result of professional APCs being the only cells that express the molecules that provide co-stimulatory signals to T cells?
Only professional APCs that are presenting the appropriate antigenic peptide can initiate a T cell response
What happens to naive T cells that encounter antigens without receiving a co-stimulatory signal?
They become anergised (unresponsive) to further antigen presentation
What does the anergisation of naive T cells that encounter antigen without receiving a co-stimulatory signal provide?
A further mechanism of tolerance to self-antigen presentation
Why does anergisation achieve a mechanism of tolerance to self-antigen presentation?
As antigen presentation outside of lymphoid tissue usually lacks co-stimulation
Draw a diagram illustrating what happens when a naive T cell encounters an antigen alone
Draw a diagram illustrating what happens if a T cell recieves a co-stimulatory signal alone
What do APCs play a major role in determining?
The type of immune response produced
Give an example of how APCs play a major role in the type of immune response produced
By influencing the balance between Th1 and Th2 cells, which biases the outcome towards a cell mediated or humoral response
What is a T cell capable of doing once it has been activated by a professional APC?
Recognising the antigen presented by any cell that expresses the same antigen/MHC complex without co-stimulation
What is the result of T cells being able to recognise the antigen presented by any cell that expesses the same antigen/MHC complex without costimulation after it has been activated?
T cells are able to reponsd to antigens presented at distant sites in the body
What do T cells undergo following activation?
A process of clonal expansion and differentiation
What happens to the T cells produced from clonal expansion and differentiation?
Some of these cells become activated T helper or cytotoxic cells, whilst others remain in a partially-activated state and recirculate as memory cells
How do memory cells differ from naive T lymphocytes?
In terms of their requirements for activation
They have less stringent requirements for activation of re-contact with antigen than naive T lymphocytes
What is the result of memory cells having less stringent requirements for activation on re-contact with antigens?
On subsequent antigen exposures, a response is developed faster
What happens in the effector phase of the adaptive immune response?
Activated cells migrate to the site of infection in order to combat infection within the peripheral tissues
What plays a central role in the effector phase of the adaptive immune response?
CD4 cells
What is CD4s role in the effector phase?
Co-ordinates the antigen-specific immune response through cytokine release
What is the effect of cytokine release in the effector phase of the adaptive immune reponse?
- B-cell isotype switching
- Affinity maturating of antibody response
- CD8 cells in the induction of effector CTL
- Enhancement of NK cell function
- Macrophage activation in delayed type hypersensitivity reactions
- Bacterial and fungal killing
What happens in the resolution phase of the adaptive immune response?
Non-specific phagocytic cells resolve any damage caused by the infection and promote healing
What non-specific phagocytic cells are involved in the resolution phase of the adaptive immune response?
- Macrophages
- Fibroblasts
