6. Antigen recognition by lymphocytes Flashcards
what is an antigen
a molecule or part of a molecule that is specifically recognized by BCRs and TCRs
stimulates an immune response
can be foreign or self
what is an epitope
the region or sites of antigen that are recognized by the immune system - bind to specific Igs or TCRs
characteristics of BCRs
known as immunoglobulins
membrane-bound form: on B-cell surface, functions as cells receptor for antigens
secreted form: aka antibody, binds pathogens in extracellular spaces, recruits other cells to destroy pathogen it has bound
structure of an antibody
2 heavy chains
2 light chains
disulphides bond joins heavy chains and light chain to heavy chain
antigen binding site at NH2 terminal
effector function at COOH terminal
weight of an antibody
heavy chain = 50kd
light chain = 25kd
2 of each
total = 150kd
avidity vs affinity of an antibody
avidity: the total strength od the interaction between the antibody and antigen
affinity: the strength of interaction between a single antigen-binding site and antigen
domains in heavy and light chains
heavy - 4 domains: 1 variable, 3 (or 4) constant
light - 2 domains: 1 variable, 1 constant
characteristics of Ig domains
each domain is 110aa in length
N-terminal has variable domain - binds antigen
C-terminal has constant domain - effector function
C-domains distinguish the different antibody classes
what is the hinge region of an antibody
lies within the constant region and joins the 2 arms
allows flexibility in binding to multiple antigens
differs between isotypes
breaks the antibody not 2 Fab and 1 Fc region
what are the Fab and Fc parts of antibodies
Fab = antigen-binding activity, 2 of them
Fc = biological activity, differs between H chain isotypes
what are distinct characteristics in the C region of antibodies that differ between isotypes
- the number and location of disulphide bonds
- the number of attached carbohydrate groups
- the number of C domains
- the length of the hinge region
which Ig is the heaviest
IgM (pentamer)
which Ig is the most abundant in blood/serum
IgG
which Igs activate complement
IgG and IgM (mostly IgM)
which Ig works in parasite immunity and allergic reactions
IgE
which Ig acts at mucosal surfaces, secreted into the gut and respiratory tract and in breast milk
IgA
which Ig has an unknown role
IgD
which Ig is responsible for passive immunity to the baby
IgG
what is a J chains
a polypeptide chain that binds Ig molecules to form polymers
allows IgM to form pentamers
allows IgA to form dimers
what forms are IgA molecules found in in the body
dimers in mucous secretions
monomers in plasma
why is it helpful that IgM is a pentamer?
individual binding sites are low affinity so having more binding sites makes up for overall functional binding strength
what are hyper variable regions (aka CDRs)
regions within the variable regions of both H and L chains that ACTUALLY contact the antigen
make up the antigen binding site
hypervariable regions vs framework regions
HV regions: 3 of them, most variable part is HV3
FR regions: regions between HV regions that provide structural framework for the Ig domain, 4 of them
what is combinatorial diversity
antibodies of different specificities are created through various combinations of H and L chain V regions
occurs during development of B cells in the bone marrow
characteristics of T cell receptors
transmembrane protein with an almost entirely extracellular structure
consists of an a and B chain (some also have y and d)
each chain has a C and V region
antigen specific - each T cell expresses one type of TCR
TCRs and their interaction with MHC/HLA
peptides must be presented to TCRs via cell-surface protein receptors on the MHC family
no MHC = no T cell activation
structure and characteristics of MHC class I
a-chain: a1, a2, a3
B-chain: B2 macroglobulin
- a1 and a2 form the peptide-binding groove
- a3 is transmembrane
- B2 is soluble
MHC I binds peptides 8-10aa in length generated from intracellular proteins and presents them to CD8 T cells
structure and characteristics of MHC II
a-chain: a1, a2
B-chain: B1, B2
- a1 and B1 make the peptide-binding groove
- a2 and B2 are transmembrane
- more important in bacterial infections
MHC II binds to peptides 13-25aa in length generated from extracellular proteins and presents them to CD4 T cells
co-receptors of MHC molecules
CD4 coreceptor: binds MHC II, targets extracellular pathogens
CD8 coreceptor: binds MHC I, targets intracellular pathogens
what does the TCR complex consist of
TCR
MHC
co-receptor (CD4 or 8)
CD3 - 6 subunits
what is the purpose of the CD3 complex
recruits signaling molecules that are activated upon TCR engagement - drives T cell activation
true or false: TCR is not expressed without CD3 because it is needed to bring the TCR to the surface
true
what is meant by “TCR’s have dual specificity”
they interact with both the antigenic peptide and the polymorphic features of the MHC molecule
characteristics of IgM
pentamer (heaviest)
first Ig produced after B cell activation
present in bloodstream, NOT tissues
interacts with C1 in complement
