L6 B Cells and Antibodies

Question 1

What does antigen-specific mean?

Answer 1

The antibody binds to a particular antigen (eg measles) with high affinity, but not to other antigens (eg pneumococcus)

Question 2

Which antibody region binds the antigen?

Answer 2

FAB region

Question 3

What antibody region interacts with other components of immunity?

Answer 3

Fc Region

Question 4

What is the FAB region?

Answer 4

Antigen-binding site (FAB) is made up of light chains and heavy chains

Because this region needs to be able to bind any potential antigen, it is very VARIABLE between different antibodies; therefore also known as the variable region

Question 5

What does the variable region recognise?

Answer 5

Epitopes of antigen

Question 6

What is the relationship between B lymphocytes and antibodies?

Answer 6

Antibodies attached to B cells at Fc region

Each B cell produces only 1 antibody: all the antibodies on its surface are the same

Each antibody (and therefore each B cell) will be specific for particular protein antigens

In addition, antibodies are secreted into the bloodstream and circulate as free proteins

Question 7

IgM [4]

Answer 7

First antibody produced in an immune response

Doesn’t bind antigen very well (‘low affinity’)

Makes up for it by forming pentamers, which are held together by a joining (‘j’) chain

As immune responses progress, the IgM response switches to other antibody isotypes

Question 8

IgA [5]

Answer 8

The only antibody that can cross mucosal surfaces

Found in secretions eg gut, tears, saliva

Forms dimers (joining ‘j’ chain)

Protected from digestion by secretory component (‘s’ chain)

Colostrum (forerunner of breast milk) is rich in IgA antibody

Question 9

IgD

Answer 9

like IgM, the first antibody produced by a B cell, but has no known function

Question 10

IgG

Answer 10

the main mature antibody form; circulates as a monomer

Question 11

IgE

Answer 11

circulates as a monomer; exact function not known, but believed to be important in parasitic infection. Definitely important in allergic disease

Question 12

What do antibodies do?

Answer 12

Bind to things and directly affect them - neutralisation, receptor blocking

Bind to things and interact with another element of immune system - phagocytes, complement, mast cell activation

When bound to B cells act as a B cell receptor

Question 13

What is neutralisation by antibody?

Answer 13

Anti-toxin antibodies bind to the toxin and neutralise it

The bacteria clostridium
Tetani releases a toxin that
causes muscle contraction
tetani

Question 14

What is receptor blocking by antibody?

Answer 14

Viruses use receptor to attach to host cell and gain entry

Antibody blocks receptor

Question 15

What is opsonisation?

Answer 15

Bacteria coated by circulating antibody

Phagocytic cells have receptors for the Fc portion of the antibody

This enhances phagocytosis by reducing the repulsion between two negatively charged membranes

This process of coating bacteria to enhance phagocytosis is called OPSONISATION

Question 16

What is mast cell activation?

Answer 16

Mast cells have surface Fc receptors

Mast cells become coated wth IgE antibody from circulation

When the appropriate antigen binds to the IgE antibodies and cross-links them, the mast cell ‘degranulates’, releasing histamine

?Important in dealing with parasitic infections

Inappropriately activated in allergy

Question 17

What is antibody-dependent cellular cytotoxicity?

Answer 17

NK cell recognises antibody-coated bacteria by Fc receptor; the target organism is then killed by non-phagocytic means.

Question 18

What is antibody diversity?

Answer 18

Each B cell produces a single antibody

Between all these B cells, antibodies must be produced that can bind to any protein

This implies a huge number of different B cells and antibodies – we each have several million

Question 19

Somatic recombination and the variable region

Answer 19

Total number of possible genes = number of segments in 1 x number in 2 x number in 3

Question 20

What are the advantages of somatic recombination?

Answer 20

Huge diversity – we can recognise whatever is in our environment
A large numbers of receptors can be made from a smaller area of DNA
Everybody has a unique repertoire – resilient in different enviroments/ against emergent pathogens
Still inherit the gene segments, so get some benefit from evolutionary experience

Question 21

What are the disadvantages of somatic recombination?

Answer 21

The receptors are generated at random - many combinations will not work out:
Some can’t fit together biochemically
Some will bind to our own proteins (self antigens)

B cells with dysfunctional receptors mostly destroyed (deleted) – process is therefore energy intensive

Deletion of B cells that can recognise self-antigens is not complete – potential for autoimmune disease

Question 22

What is clonal selection?

Answer 22

We all start with a unique set of B cells, each producing a single antibody; small number of each B cell

During infection, those with the best response to the infection antigen are selected out

Known as ‘clonal selection’ – division and selection of the ‘fittest’ B cells

Question 23

Affinity maturation

Answer 23

Class switch: IgM in the ‘primary response’ switches to IgG. The variable region of the antibody remains the same

Somatic hypermutation: Random mutations are introduced into the variable region of the antibody, so the daughter cells produce a slightly different antibody. Further rounds of clonal selection pick out the best receptors

Question 24

Memory of the immune system

Answer 24

The best B cells to deal with a particular infection have now been selected
and their number increased. These mature B cells become long-lived memory cells. When stimulated again, they respond vigorously

This is the basis of the secondary immune response