4. Antibody structure and function

Question 1

What are antibodies?

Answer 1

Soluble proteins that are found in blood and secretory fluids e.g. mucus + milk that bind specifically to foreign material antibodies

Question 2

What are antibodies produced by?

Answer 2

B lymphocytes, form the B cell antigen specific receptor

Question 3

What are antibodies collectively known as?

Answer 3

Immunoglobulin (Ig)

Question 4

What are features of antibodies?

Answer 4

Most important features are specificity and biological activity (bi-functional)

• variable region binds antigenic structure
• biological activity is restricted to the constant region of the heavy chain

Question 5

What is the antibody domain structure?

Answer 5

V region binds antigen 
C region of heavy chain (biological function)
1. Oponisation 
2. Activate complement 
3. FcR mediated activity

Question 6

What is the basic structure of an antibody?

Answer 6

Composed of two chains.
- light and heavy
- which are comprised variable and constant regions
Variation is not over the entire region of the variable domain, mostly restricted to three regions within N-terminal of both H and L chains - HYPERVARIABLE COMPLEMENTARY - DETERMINING REGION (CDR)

Question 7

What is antibody affinity?

Answer 7

measure of the strength of a single antibody-antigen (single FaB with Ag epitope)

Question 8

What is antibody avidity?

Answer 8

measure of the strength of the interaction of an antibody with an antigen at multiple sites

Question 9

How is diversity of the variable light (k) chain produced?

Answer 9

40 (V) variable + 5 (J) joining gene segments

1V and 1J makes a whole variable domain therefore recombinational diversity is 40x5 = 200

Question 10

How is diversity of the heavy chain produced?

Answer 10

5 different classes (isotypes) - IgM, IgG, IgA, IgD, and IgE
- differ in the structure of the H chain
Structural differences confer biological function for each isotype (what happens after binding antigen)

Question 11

What are the Ig classes and subclasses?

Answer 11

Distinguished by structure of constant region of the heavy chain 
Effector functions of each class results from interactions between its heavy chain constant region and other serum protein or cell membrane receptor 
IgA found as two subclasses (IgA1 + IgA2)

Question 12

How are the subclasses ordered into structural classes?

Answer 12

IgG, IgD, IgE = monomer
IgA = monomer, dimer
IgM = Pentamer

Question 13

What is the biological activity of IgE?

Answer 13

Mediate the immediate hypersensitivity

Question 14

What is the biological activity of IgG?

Answer 14

80% of the total serum Ig - most abundant, expressed on the surface of B cells

Question 15

What is the biological activity of IgM?

Answer 15

5-10% of total serum Ig - expressed on the surface of B cells

Question 16

What is the biological activity of IgD?

Answer 16

Co-expressed on surface of B cells with IgM - function is unknown

Question 17

What is the biological activity of IgA?

Answer 17

Predominant Ig expressed in external excretions - prevents attachment of pathogens to external surfaces

Question 18

What are AB effector functions?

Answer 18

Refer to what AB’s do after they contact Ag - some common effector functions some specific for antibody isotope

Question 19

What are effector functions of IgG?

Answer 19

Neutrilisation of toxin e.g. tetanus, snake and scorpion venom - by blocking their active sites
- binds with antigenic determinants present on various portions of the protein coat inc. region used by virus for attachment to the target cell

Question 20

What are effector functions of IgA?

Answer 20

Primary immunoglobulin involved in defence against local infection in respiratory or gastrointestinal tract
- prevents invading organisms from attaching to and penetrating epithelial surface
- not complement activating or fixing
Secretory IgA is an effective antiviral antibody: prevents virus entering host cell
- Good agglutinating antibody

Question 21

What are effector functions of IgE?

Answer 21

Eosinophils attacking schistosome larva

• large parasites e.g. worms cannot be ingested by a phagocyte
• but when worms are coated with IgE, eosinophils can attack using IgE Fc receptors

Question 22

What are effector functions of IgM?

Answer 22

Good agglutinator because of pentameric form especially if pathogen have repeated pattern of same antigenic determinant

• excellent complement - fixing or complement activating antibody
• not good at neutrilizing toxins or viruses

Question 23

How does IgE form a cross-linking mast cell surface?

Answer 23

Mast cells have high affinity IgE Fc receptors on their surface that are occupied by IgE molecules
Antigen cross-linking of bound IgE crosslinks the Fc receptors - triggers degranulation of mast cell and release of inflammatory mediators (histamine)
- symptoms of hayfever
- called type 1 hypersensitivity