Lecture 4 ((3) - Week 2B) Flashcards
The immune system must protect us against a wide variety of pathogens
eg bacteria, viruses, parasites, fungi
• which pathogens an individual will encounter is not known in advance
• therefore, immune system must maximize its potential to respond to diverse challenges
The immune system must
• recognize and respond to pathogens
(failure = death from infectious disease)
• but not respond to components of our own bodies
(failure = auto-immune disease)
ie must discriminate “non-self” from “self”
Antigen
anything that binds to a specific receptor on T or B cells
• generates antibody
• can be bits of bacteria, viruses etc (foreign antigen)
• pathogens can also be bits of our own bodies (self antigen)
Epitope
the portion of the antigen that is recognized and bound by a receptor on an immune cell
• structure of antigen that’s recognized
• numerous
• antigens contain many epitopes
Complex antigens (eg proteins) can contain
multiple different epitopes, each of which can be recognized by different antibodies
Epitopes recognized by antibodies are
often shapes formed by the way proteins fold
• different antibodies “fit” different shapes
Cells of innate and adaptive immune systems recognize
non-self in different ways
Innate immune system
- NK cells
- monocytes/macrophages
- dendritic cells
- granulocytes
- a limited number of common microbial structures can be recognized: PAMPs (toll-like receptors)
Adaptive immune system
- T cells
- B cells
- can recognize MANY antigens
Overview of antigen recognition
- receptor binds to Ag
- signalling cascade initiated (to nucleus, tell cells to start division)
- gene transcription
- production of effector molecules (eg antibodies/cytokines)
Signalling cascade
–>
cell starts dividing
B cells
make antibodies
Antibody is also known as
immunoglobulin (Ig)
Recognition of antigen by B cells
- B cells recognize antigens using the B cell receptor (BCR)
- the BCR is an antibody molecule anchored to the B cell - transmembrane
- antigens are recognized directly (they don’t require presentation by other cells)
- epitopes are often conformational (shapes formed by protein folding)
• T cells recognize antigen = complex
• antibody molecules on B cell surface BIND native antigen
–shape and conformation (wrong shape = no binding = no action)
In mamamls, B cells develop in
bone marrow stem cell --> pro- B cell --> pre- B cell (start to express antibody on surface)--> immature naive B cell --> mature naive B cell
• naive = hasn’t seen antigen
Each new B cell
has a unique receptor
• recognizes a different antigen
Each B cell expresses
only 1 type of receptor
• recognize only 1 antigen
Recognition of antigen by B cells
- activated B cells differentiate into plasma cells - secrete a soluble form of the receptor - antibody
- the antibody recognizes the same antigen as the BCR
• B cell divides, loses antibody from surface –> makes a lot of antibody in solution (not on surface)
Activated B cells differentiate into
plasma cells that secrete a soluble form of the receptor (antibody)
Plasma cells secrete
a soluble form of the receptor (antibody)
Each antibody recognizes
a different antigen
• Ag = antigen
• Ab = antibody
Structure of a typical antibody
Y shaped Arms of the Y • light chain on outside of each arm • heavy chain on inside of each arm • variable region is distal and made of a combination of shapes of 2 identical heavy (VH) and 2 identical light (VL) • antigen binds the variable region
The stem of the Y
• constant region
• controls what the antibody does
• heavy chain = γ α μ ε δ
there are 2 types of light chain and 5 types of heavy
2 types of light chain
- kappa κ
* lambda λ
5 types of heavy chain
- mu μ
- gamma γ
- alpha α
- epsilon ε
- delta δ
Variable domains
contains the sequences that interact with antigen
Variable regions of H and L chains
combine to form shape that fits that of the antigen
Fab
arms of the Y
binds antigen
Papain digestion (each separately) Pepsin digestion ( F(ab')2 both parts separated from Fc)
F(ab’)2
(2 = subscript)
(unsure)
the arms of the Y together
Fc
determines function of antibody
crystallizes
V domains contain
areas of hypervariability
• HV regions are also known as complementarity determining regions (CDRs) - 6 combine to form each antibody binding site
HV regions
- hypervariable regions
- also known as complementarity determining regions (CDRs)
- 6 combine to form each antibody binding site
Antibody classes
determined by the constant region of the heavy chain
- IgG = γ heavy chains
- IgE = ε heavy chains
- IgA = α heavy chains
- IgD = δ heavy chains
- IgM = μ heavy chains
all have 2 heavy and 2 light chains, but IgM has 10 heavy and 10 light chains, IgA can have 4 heavy and 4 light chains
IgG =
γ heavy chains
2 heavy 2 light
4 subclasses
IgE =
ε heavy chains
2 heavy 2 light
IgA =
α heavy chains
2 heavy 2 light
Or another Y to get 4 heavy and 4 light
2 subclasses
IgD =
δ heavy chains
2 heavy 2 light
IgM =
μ heavy chains
10 heavy, 10 light
Different classes have
different effector functions
IgG has
4 subclasses in humans • IgG1 • IgG2 • IgG3 • IgG4
IgA has
2 subclasses
• IgA1
• IgA2
B cells can produce
different classes of antibodies • IgM and IgD --> • IgM • IgG • IgA • IgE
- IgD = recognition
- IgE = allergies
- ER makes protein
B cells receptor diversity
- millions of different antigens can be recognized
- each new lymphocyte expresses only 1 type of receptor and recognizes only 1 antigen
- lymphocytes with any given specificity are rare
- an antigen will only activate those lymphocytes with the “right” receptor
- these lymphocytes divide repeatedly - daughter cells express the same receptor
- a large number of useful cells are generated (clonal expansion)
A new lymphocyte expresses
only 1 type of receptor and recognizes only 1 antigen
Lymphocytes with any given
specificity are rare
An antigen will only activate those lymphocytes with
the “right” receptor
Lymphocytes divide repeatedly
clonal expansion
• daughter cells express the same receptor
• a large number of useful cells are generated
How is receptor diversity generated?
- each developing B cell expresses a distinct receptor
- tens of millions of receptors required - how?
- a gene for each receptor? - no, would require tens of millions of genes
- instead, diversity is generated by “mixing and matching” gene segments within the heavy chain and light chain loci
Diversity is generated by
random “mixing and matching” of variable, diversity, and joining segments
Diversity is generated by random “mixing and matching” of
variable, diversity, and joining segments
Making an immunoglobulin heavy chain
- D-J joining: DNA spliced between diversity and joining regions
- V-DJ joining: DNA spliced out between V and DJ
- transcription
- RNA splicing –> chooses constant part and makes different variants
• mediated by RAG recombinase enzymes
Somatic DNA recombination generates
antibody diversity
… generates antibody diversity
somatic DNA recombination
Antibody diversity
2 processes act during B cell development
• combinational diversity
• junctional diversity
~ 5x10^13 antibodies
Combinational diversity
mixing and matching of V, (D), and J segments
• different combinations of H and L chains
Junctional diversity
(CDR3)
• addition of P- and N- nucleotides at joins
Antibody responses are dynamic
they evolve DURING immune response
Primary = first time see antigen
• IgM levels in serum up then down
• IgG up
Secondary = second time to see antigen
• IgG down then up
isotype switching and and affinity maturation
Antibody responses are dynamic - they evolve during an immune response
- class (isotype) switching
2. affinity maturation
Class switching
- a given B cell starts by making IgM (and IgD) but can switch to making IgG (or IgA or IgE)
- class switch recombination allows a given V-D-J unit to be associated with different constant regions
- antibody specificity remains constant whilst biological effector functions are varied
IgM first, then switches primary
secondary made IgG
Class switch recombination
IgM variable (VDJ) followed by constant
• activation induced cytidine deaminase (AID) = DNA chopped out
IgG made
• take out genes that make A and put in genes that make B
• different constant region
Affinity maturation
improves the quality of antibodies during an immune response by increasing their affinity for antigen
(ie makes the antigen binding site a better fit for the antigen)
• changes variable regions
• affinity = strength of interaction
Affinity maturation mechanism
by somatic hypermutation
• individual nucleotides in variable regions are randomly replaced with alternatives
• “tested” against antigen displayed on follicular dendritic cells (FDC) - many mutations are useless
• B cells with the best fit antibodies survive
occurs during an immune response, not during B cell development`
Affinity maturation and class switching occur in
germinal centers (GC) in lymph nodes and spleen
• GC form 3-4 weeks after initial antigen exposure
• rapid initial response is followed by an improved, more effected and diversified, response
• antigen sticks on surface and presents in follicular dendritic cells
What do antibodies do to protect us against infections?
five things:
- agglutination
- neutralization
- opsonisation
- complement activation
- antibody dependent cellular cytoxicity (ADCC)
Agglutination
crosslinking by antibodies creates larger particles that are taken up more efficiently by phagocytes
• esp IgA antibody
• multivalvent antibodies and antigens
Neutralization
can block attachment of pathogens to receptor on cells and block the action of bacterial toxins
• keeps shit out
• antibodies bind to pathogens so can’t fit into receptor on target cell
Opsonization
FcR mediated uptake enhances pathogen clearance by macrophages and neutrophils
• FcR = receptor for Fc part of antibody
-receptor for heavy chain
• constant part of antibodies attaches to receptor on target cell so pathogens/antigens can’t get in
Complement activation
- “trained spotters”
- proteins in blood
- antibody binds to surface of cell enzyme splits C3 into C3a and C3b
- C3b + C5 into C5 convertase to make MAC (membrane attack complex)
- punches hole in cell –> lysis
• classical, alternative, lectin pathways
Antibody dependent cellular cytoxicity (ADCC)
role in killing:
• virally infected cells, helminths, tumour cells
• may be utilized for therapies
• antibody bound to cell
• NK cell or macrophage comes along, recognizes, makes molecules that kill
- viral peptides stick out of target cell surface
- antibodies bind, constant (Fc) region sticks out
- NK cell or macrophage hasFc receptor
- meet up, macrophage/NK cell releases molecules to kill infected cell
Antibody classes and subclasses have
different biological properties
B cells
make antibodies
Each B cell expresses
a single unique receptor for antigen
Somatic recombination contributes to
receptor diversity during B cell development in bone marrow
During an immune response, antibodies
improve and diversify - affinity maturation an isotype switching
Antibodies have 5 main biological activities that
help fight infections
