6. Antigen recognition by lymphocytes

Question 1

what is an antigen

Answer 1

a molecule or part of a molecule that is specifically recognized by BCRs and TCRs
stimulates an immune response
can be foreign or self

Question 2

what is an epitope

Answer 2

the region or sites of antigen that are recognized by the immune system - bind to specific Igs or TCRs

Question 3

characteristics of BCRs

Answer 3

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

Question 4

structure of an antibody

Answer 4

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

Question 5

weight of an antibody

Answer 5

heavy chain = 50kd
light chain = 25kd
2 of each
total = 150kd

Question 6

avidity vs affinity of an antibody

Answer 6

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

Question 7

domains in heavy and light chains

Answer 7

heavy - 4 domains: 1 variable, 3 (or 4) constant
light - 2 domains: 1 variable, 1 constant

Question 8

characteristics of Ig domains

Answer 8

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

Question 9

what is the hinge region of an antibody

Answer 9

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

Question 10

what are the Fab and Fc parts of antibodies

Answer 10

Fab = antigen-binding activity, 2 of them
Fc = biological activity, differs between H chain isotypes

Question 11

what are distinct characteristics in the C region of antibodies that differ between isotypes

Answer 11

• the number and location of disulphide bonds
• the number of attached carbohydrate groups
• the number of C domains
• the length of the hinge region

Question 12

which Ig is the heaviest

Answer 12

IgM (pentamer)

Question 13

which Ig is the most abundant in blood/serum

Answer 13

IgG

Question 14

which Igs activate complement

Answer 14

IgG and IgM (mostly IgM)

Question 15

which Ig works in parasite immunity and allergic reactions

Answer 15

IgE

Question 16

which Ig acts at mucosal surfaces, secreted into the gut and respiratory tract and in breast milk

Answer 16

IgA

Question 17

which Ig has an unknown role

Answer 17

IgD

Question 18

which Ig is responsible for passive immunity to the baby

Answer 18

IgG

Question 19

what is a J chains

Answer 19

a polypeptide chain that binds Ig molecules to form polymers
allows IgM to form pentamers
allows IgA to form dimers

Question 20

what forms are IgA molecules found in in the body

Answer 20

dimers in mucous secretions
monomers in plasma

Question 21

why is it helpful that IgM is a pentamer?

Answer 21

individual binding sites are low affinity so having more binding sites makes up for overall functional binding strength

Question 22

what are hyper variable regions (aka CDRs)

Answer 22

regions within the variable regions of both H and L chains that ACTUALLY contact the antigen
make up the antigen binding site

Question 23

hypervariable regions vs framework regions

Answer 23

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

Question 24

what is combinatorial diversity

Answer 24

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

Question 25

characteristics of T cell receptors

Answer 25

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

Question 26

TCRs and their interaction with MHC/HLA

Answer 26

peptides must be presented to TCRs via cell-surface protein receptors on the MHC family
no MHC = no T cell activation

Question 27

structure and characteristics of MHC class I

Answer 27

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

Question 28

structure and characteristics of MHC II

Answer 28

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

Question 29

co-receptors of MHC molecules

Answer 29

CD4 coreceptor: binds MHC II, targets extracellular pathogens
CD8 coreceptor: binds MHC I, targets intracellular pathogens

Question 30

what does the TCR complex consist of

Answer 30

TCR
MHC
co-receptor (CD4 or 8)
CD3 - 6 subunits

Question 31

what is the purpose of the CD3 complex

Answer 31

recruits signaling molecules that are activated upon TCR engagement - drives T cell activation

Question 32

true or false: TCR is not expressed without CD3 because it is needed to bring the TCR to the surface

Answer 32

true

Question 33

what is meant by “TCR’s have dual specificity”

Answer 33

they interact with both the antigenic peptide and the polymorphic features of the MHC molecule

Question 34

characteristics of IgM

Answer 34

pentamer (heaviest)
first Ig produced after B cell activation
present in bloodstream, NOT tissues
interacts with C1 in complement