Lecture 2: B Cells and Generation of Antibodies Flashcards
Basic Structure of Ig
- Each molecule of immunoglobulin contains two identical heavy chains and two identical light chains (H2L2)
- Each chain has a constant region and a variable region
- The antigen binds to the antibody at hypervariable regions of the VH and VL
- The class of the molecule is determined by the heavy chain constant region
Heavy Chain
One type of protein chain in an immunoglobulin molecule
There are several different classes of heavy chains (A,D,E,G,M) which confers a functional activity on the antibody molecule
Each immunoglobulin contains two identical heavy chains
Heavy Chains are made up of 3 constant regions and 1 variable Region
Isotype
An antibody class which is determined by the constant region of the heavy chain
Heavy chains are classified as alpha, delta, epsilon, gamma, and mu
The heavy chain deterines the isotype
IgA-alpha
IgD-delta
IgE-epsilon
IgG-gamma
IgM-mu
Light Chain
The smaller polypeptide chain that makes up an immunogloulin molecule
Consists of one V and one C domain
Disulfide bonded to the heavy chain
There are two isotypes of light chains known as kappa and lambda (determined by the constant region of the light chain)
Domain
An Ig Domain is 110 amino acids with an intradomain disulfide bond
V Domain
The amino terminal potein domain of the polypeptide chains of immunoglobulins
The most variable part of the chain
VL and VH
C Domain
The part of an immunoglobulin protein chain that is relatively constant in amino acid sequence between different molecules
The constant region of the antibody determines its particular effector function
CL, CH1, CH2, CH3
Ig Hypervariable Region
The complementarity determining regions of the heavy and light chain variable domain form the antibody binding site
The most variable region of the V domain
There are 3 CDR regions in each V domain
CDR
Complementarity Determining Region
Most of the differences among antibodies fall within the CDR
It is the CDRs on both light and heavy chains that constitutes the antigen binding site of the antibody molecule
Fc Region
The c terminal domain of the constant region of the antibody
Fc Receptor
The cell surface receptor specific for the Fc portion of certain classes of immunoglobulins
Present on lymphocytes, mast cells, macrophages, and other accesory cells
Classes of Ig
- IgA
- IgD
- IgE
- IgG
- IgM
*differ in the constant region of the heavy chain
IgA
The class of immunoglobulin charachterized by alpha heavy chains
Can occur in monomeric and dimeric form
Dimeric IgA is the main antibody secreted by mucosal lymphoid tissues
Major antibody in secretions such as saliva, tears, and breast milk
Plays a primary role in protection of pathogens that invade the gut or respiratory mucosal
A j chain holds the molecules together and a secretory component bridges the two IgA molecules in a process of transcytocysis from the extracellular fluid into the lumen
IgD
The class of immunoglobulin charachterized by delta heavy chains.
Appears as surface immunoglobulin on mature B cells but its function is unknown
IgE
The class of immunoglobulin charachterized by epsilon heavy chains.
Involved in the defense against parasite infections and allergic reactions.
IgE does not have to be agregated to bind to the Fc receptor, but once antigen binds agregation occurs
Found on basophils and mast cells which have an IgE specific receptor
IgG
The class of immunoglobulin charachterized by gamma heavy chains
The most abundent class of immunoglobulin in the serum
Crosses the placenta and represents the serum antibody of newborns
IgG antibodies can bind antigen and the complex is taken up by macrophages or neutrophils through an IgG specific Fc receptor (the antigen must have an agregation of IgG on the surface to activate binding) and is then killed
IgM
The class of immunoglobulin charachterized by mu heavy chains
Found as a monomer on B cells and is secreted by plasma cells as a pentamer
First immunoglobulin to appear on the surface of B cells and the first to be secreted - First serum antibody made in a primary immune response
Transcytosis
IgA undergoes transcytosis where An IgA dimer binds to a polyimmunoglobulin receptor (pIgR) on the extracellular fluid side of an endothelia cell
Once bound to the Fc receptor, the IGA dimer is taken into the cell and passed into the lumen through transcytosis, leaving a secretory component from the pIgR
Organization of Ig H and Ig L genes and rearangements to produce heavy and light chains
During B cell development DNA rearangment occurs and is mediated by RAG proteins
V and J segments are chosen and spliced together to form the light chain
V, J, and D segments are chosen and spliced togeteher to form the heavy chain
B Cell Development
B cells develop in the bone marrow
- Common lymphoid progenitor
- Pro-B cell - intracellular mu chain
- Pre-B cell - mu chain is on the surrface of the cell, and a surrogate L chain is present
- Immature, naive B Cell- the surrogate L chain is replaced by the correct L chain
Immature B cells leace the bone marrow and develop into mature naive B-cells which are waiting for the antigen while circulating through the periphrial lymphoid organs
Affinity Maturation
Following the introduction of an antigen, the VDJ heavy chain and the VJ light chain undergo a somatic hypermutation and high affinity antiboies are selected
This requires the enzyme AID - activation induced cytidine deaminase
Upon initial exposure to the antigen, antibody binding does not have a high affinity, however through the process of somatic hypermutation the Kd value devreases and infinity increases.
Somatic hypermutation occurs most often at the antigen binding site (CDR1, CDR2, and CDR3)
Antibody diversity generation
A large antibody repritoire is developed through gene rearrangments and somatic hypermutation
Combinatoral Joining
A large heavy chain repertoire develops by combinatorially joining any V gene segment with any D gene segment with any J gene segment
A large light chain repertoire develops by combinatorially joining any V gene segment with any J gene segment
The combination of any heavy chain with any light chain results in a large antibody repertoire
Allelic Exclusion
Only one of the two IgH and IgL aleles is rearranged functionally so that one B cell makes only a single H and L chain
VDJ gene rearangment
DNA gene rearangments occur during B cell development
In the common lymphoid progenitor, one D segment is selected to join with one J segment on the heavy chain
The DJ segment then joins with one V segment and the heavy chain migrates to the surface of the Pre-B cell where is is joined by surogate light chains. The surrogate light chains then send a signal back to the cell to stop rearrangment of the heavy chain
The immature B cell still in the bone marrow then undergoes VJ rearangment of either the kappa or lambda light chain which replaces the surrogate light chain sending a signal back to the cell to stop rearrangment
*DNA cleavage is mediated by RAG enzyme
Defficency in RAG enzyme
A defficincy in the RAG enzyme would prevent B cells and T cells from forminh and would result in an immune defficent animal.
Isotype switching
Durring the secondary immune response IgM bearing B cells switch to produce another isotype such as IgG, IgE, or IgA
This occurs through DNA deletion by cutting and rejoining of DNA near switch sequences
Isotype switching is not RAG dependent, it requires the enzyme AID
