Lecture 2 - Biology Of B Cells

Front 1

What is the structure of antibodies?

Back 1

-composed of 2 heavy chains and 2 light chains, connected by disulphide bridges
-one heavy & one light chain combine to form an antigen binding site at the N-terminus of both chains
-each antibody has 2 antigen binding sites which allow the antibody to cross link two-antigens together

LIGHT CHAIN: has a variable domain, binds antigen, and a constant domain which does not (there are 2 constant domains in genome, kappa or lambda)

HEAVY CHAIN: 1 variable domain, binds antigen, and 3 constant domains, do not bind antigen
-the different constant parts o the Ab give rise to different isotypes
-the major isotypes are IgG, IgA, IgM, IgE & IgD

Front 2

What is the clonal selection theory?

Back 2

Each B cell has a specific antibody as a cell surface receptor. The arrangement and generation of antibody genes occurs prior to any exposure to antigen. When a soluble antigen is present, it binds to the antibody on the surface of B cells that have the correct specificity. These B cell clones develop into antibody-producing plasma cells or memory cells. Only B cells, which are antigen-specific, are capable of secreting antibodies.

Front 3

How is antibody diversity generated?

Back 3

-through somatic gene rearrangement

Front 4

Describe the generation of the antibody heavy chain.

Back 4

HEAVY CHAIN
-3 gene segments (V, D, and J) are combined to create the variable domain of the B cell heavy chain or TCR β chain
-VDJ rearrangements in DNA produce the diversity of heavy chain variable domains
-first 1 D segment is attached to a J segment through removing the intervening sequence
-next the DJ sequence is attached to one of the VH segments, this is irreversible ie the missing piece of DNA can never be recovered
- μ or δ constant domains are added to complete the heavy chain by splicing the RNA (ie. removing the intron) → this is transcribed into mRNA

Front 5

Describe the generation of the antibody light chain.

Back 5

LIGHT CHAIN
-VJ rearrangements in DNA produce the diversity of light chain variable domains
-first one J segment is attached to a V segment through removing intervening sequences of DNA
-κ or λ constant domains are added to complete the light chain by splicing the RNA (ie. removing the intron)

Front 6

What happens if an animal is deficient in the enzymes responsible for VDJ rearrangements?

Back 6

-these enzymes are DNA dependent protein kinases, RAG-1 & RAG-2
-it results in severe combined immunodeficiency (SCID) whereby no B cells (or T cells) are produced
-seen in Arabian foals, homozygous deficiency
-can’t recombine B & T cell receptors = no adaptive immunity, can only survive in germ free conditions

Front 7

There are 2 possible heavy chains & 4 possible light chains (1 κ from mom, 1 κ from dad, 1 λ from mom, 1 λ from dad), yet B cells have antibodies with one specificity, what makes this possible?

Back 7

-once a functional product has been made, the homologous chromosome is inactivated (allelic exclusion)

Front 8

Describe the development of B cells? What is class switching?

Back 8

-originate from haematopoietic progenitor cells in bone marrow
-B cells become Pro-B cells upon rearrangement of heavy chain
↓
-the rearranged heavy chain is expressed in association with a surrogate light chain on the surface of the B cell, which is now called a Pre-B cell
-this occurs in the bone marrow and it is at this level that allelic exclusion occurs
↓
-light chain gets rearranged to produce an immature B-cell
↓
-these cells can now undergo differential mRNA splicing to produce both IgM & IgD
-these cells are now called mature B cells and migrate into peripheral lymphoid organs
↓
-upon encountering of antigen that binds to the surface Ig the B cell gets activated
-leads to differentiation into IgM producing plasma cells = massive increase in B cells or clonal expansion
-some B cells become memory B cells of various isotypes while others go on to produce and secrete antibodies of various isotyopes
-the process of changing from one isotype to another is called class switching

Front 9

What is primary immune response? What is secondary immune response and why is it more efficient?

Back 9

Primary immune response – first time the immune system encounters antigen
Secondary immune response – immune system encounters antigen for a second time
-B cells produce a faster immune response, antibodies with higher affinity & better functionality

Front 10

Why is class switching important and what causes class switching?

Back 10

-important in the humoral immune response because it allows the immune system to assign different functions to the antibodies by expressing them with a different heavy chain
-some antibodies will be more appropriate in dealing with certain pathogens compared to others
-the decision as to which isotype is produced is regulated by cytokines and T cells influence the outcome of which isotype is produced

Front 11

How does class switching occur?

Back 11

-occurs through a DNA looping mechanism followed by a recombination event
-following this recombination event, a circular DNA fragment is excised from the genome
-another recombination event can happen later, BUT it is impossible to recover what has been previously excised
-thus class switching is a one way event that cannot be reverted
-as a result, once a certain isotype is produced by a B cell it is not possible to go back upstream isotypes but it is possible to continue to switch to an isotype downstream in the DNA sequence

Front 12

What are polyclonal & monoclonal antibodies?

Back 12

Polyclonal – collecting serum from an animal that has been immunized against an antigen will result in a large number of different antibodies against the antigen that will not only recognize different epitopes but will be different isotypes

Monoclonal – purifying the antibodies of one B cell in order to have control of specificity and isotype of the antibody

Front 13

What are some examples of veterinary applications of monoclonal antibodies?

Back 13