L16: Antibodies - structure, function, classes, Fc receptors

Question 1

Subclasses of IgG and IgA in humans and mouse

Answer 1

Human: IgG1/G2/G3/G4 and IgA1/A2
Mouse: IgG1, G2a/G2b, G3

Question 2

IgA

Answer 2

Dimer, transport mechanisms, front-line defence mechanism and mucosal secretion, transport across epithelium

Question 3

IgG

Answer 3

Small molecule, can diffuse and cross placenta for newborn protection

Question 4

IgE

Answer 4

High affinity for mast cells and lines blood cells and connective tissues

Question 5

IgM

Answer 5

Restricted to circulatory system, pentamer

Question 6

General antibody functions (5)

Answer 6

1. Neutralisation
2. Opsonisation
3. Complement activation (opsonisation, direct killing, recruitment)
4. ADCC (antibody-dependent cell-mediated cytotoxicity)
5. Triggering of mast cells, basophils and activated eosinophils

Question 7

Neutralisation

Answer 7

Doesn’t require Fc region, binds to epitope to prevent pathogen from delivering signal

Question 8

Opsonisation

Answer 8

cluster of Fc regions bind to Fc receptors on macrophages:

1. Bacterium is coated with complement and IgG antibody
2. C3b binds to CR1 and antibody binds to Fc receptor, bacteria are phagocytosed
3. Macrophage membranes fuse, creating a membrane-bounded vesicle, the phagosome
4. Lysosomes fuse with these vesicles, delivering enzymes that degrade bacterium

Question 9

ADCC

Answer 9

Mediated by NK cells which act through downregulated MHC1 or ADCC pathway. Fab region binds to target cell where NK cells recognise Fc regions of antibodies through Fc receptors.

1. Antibody binds antigens on surface of target cells
2. Fc receptors on NK cells recognise bound antibody
3. Cross-linking of Fc receptors signals NK cell to kill the target cell
4. Target cell dies by apoptosis

Question 10

Mast Cell Activation

Answer 10

Occurs through Fc epsilon receptors which bind to IgE; potent inflammatory cells and secrete performed inflammatory mediators

Question 11

Thymus-dependent or independent antigens

Answer 11

Thymus dependent (Th cells provide signal)
Signal 1: antigen binding to B cell receptor
Signal 2:
a) TI antigen: intrinsic second signal
b) TD antigen: cytokines and CD40L

Question 12

BCR Gene Rearrangment (4)

Answer 12

1. Occurs during development of naive B cell
2. Occurs in bone marrow
3. Antigen-independent
4. Similar process occurs for TCR

Question 13

Class switching (4)

Answer 13

1. Occurs during mature IgM+ B cell activation
2. Occurs in secondary lymphoid organs
3. Antigen-dependent
4. Unique to B cells

Question 14

Somatic recombination in B cells

Answer 14

1. Early B cells - generation of diversity of BCR occuring in bone marrow
2. Mature B cells - isotype switching occuring in secondary lymphoid organs

Question 15

Mechanisms of isotype switching (3)

Answer 15

1. AID: activation-induced cytidine deaminase; conversion of cytosine to uridine
2. UNG: uracil-DNA glycosylase; locks off the base, leaves single sugar
3. APE1: apurinic/apyrimidinic endonuclease 1: removes sugar so ribose is open and nicked, enabling splicing out and ligation of DNA

Question 16

Isotype/class switching signal failing

Answer 16

Requires Th cell signals: CD40L and cytokines. If signals don’t work due to genetic defects: Hyper-IgM syndome (high IgM, low levels of Igs) results in infection susceptibility. Syndrome can result in deficiency in Th cell function or B cell machinery

Question 17

Link between IgG sub-classes & Th1/Th2 responses: Intracellular vs Extracellular pathogen control

Answer 17

Th1 responses: resistance to intracellular pathogens (macrophage activation)
Th2 responses: susceptibility to intracellular pathogens (macrophage deactivation), resistance to large extracellular pathogens (allergy-type response)

Question 18

Link between IgG sub-classes & Th1/Th2 responses: Macrophage regulation

Answer 18

Th1:
1. IgG2a - interacts with FcyR1, activating receptor and macrophage activation
2. IFN-y cytokine, increases FcyR1 expression, more tissue damage, effective intracellular pathogen celarance
Th2:
1. IgG1 - interats with FcyR2B, inhibitory receptor, macrophage deactivation
2. IL-4 cytokine, increases FcyR2B expression, less tissue damage, ineffective intracellular pathogen celarance