Generation of Antibody Diversity Flashcards
How can the immune system produce million of different molecules when the genome (of humans contained only 22,000 genes?
B and T cells have a mechanism in which rearrangement and recombination are possible to generate diversity
What is V (D) J recombination?
-mechanism by which lymphocytes create unique antigen-specific receptors
**mediated by V (D) J recombinase
_WITHOUT RAG1 OR RAG2 B cells NOR T cells CAN BE MADE!!!! (in recombination)
What are the steps of V (D) J recombination?
(DNA is cut and ligated together)
-in DNA genome, one V segment is rearranged with one J segment
-it is then TRANSCRIBED TO RNA
- THEN introns are SPLICED out
-spilcing results in Mature messenger RNA what has C-J-C segments that are ULTIMELTY TRANSLATED to a light chain polypeptide
What is a light chain polypeptide made of?
both V and J segments
what is Light Chain gene rearrangement?
-Coding of variable region domain of light chain (resides in N terminal) of two separate gene segments (variable ‘V’ segment and joining ‘j’ segment).
–the segments then come together in the genome of a DIFFERENTIATING B lymphocyte to create a V-J unit TOGETHER with the C region creating the entire Ig light chain.
Each VL gene produced (in light of chain rearrangement) has a ______ that is the leader sequence, which codes for the _____ that functions to target the translated polypeptide to the membrane of the cell
5’exon
signal peptide
how does light chain gene rearrangement differ in mice?
-instead of the V and J segment coming together in the differentiating B, the differentiating b lymphocyte has to PHYSICALLY join them together,
what are the two isotopes of antibody light chain? and what are their two major domains?
kappa
lambda
***either kappa or lambda, BUT never both!
major domains:
-variable region (VL)
-constant/crystallized region (CL)
What is heavy chain gene rearrangement?
- coding of variable region of heavy chain done by THREE GENE SEGMENTS (V+D+J)
*** leading to TWO REARRANGEMENTS)
-D joins J
- THEN V joins DJ
5 exon is also present as a leader sequence to target the translated polypeptide to the membrane cell
- CONSTANT REGION IS also coded (in contrast to light chain)
* after V-D-J is made, it is transcribed and than translated through ALTERNATIVE SPLICING to yield IgM or IgD
What is alternative splicing?
in heavy chain rearrangement:
- CONSTANT REGION IS also coded (in contrast to light chain)
* after V-D-J is made, it is transcribed and than translated through ALTERNATIVE SPLICING to yield IgM or IgD
**resultant B cell EXPRESSES antibodies for both Mu and delta WITH IDENTICAL ANTIGEN SPECIFICITY
What is germ line diversity?
An individual has multiple DIFFERENT V,D, J rearrangements AND multiple ALLELES for each of these genes
What is combinatorial diversity? happens I primary lymphoid organs
refers to all the different V (D) J rearrangements that occur during the differentiation of lymphocytes
-these combinations contribute to creating different heavy and light chains .
TO DETERMINE the value of chains that can be created: , multiple the V, D, and J values.
for the number of Antigen binding sites, multiple the number of heavy chains with the number of light chains
What is junctional diversity? happens in primary lymphoid organs
-imprecise joining of V,D, and J gene segments during V (D) J rearrangement
the involvement of ENZYMES involved in CHEWING the ends of joining segments and adding nucleotides back to it (in a random manner that creates further variability)
What is somatic hypermutation? happens in SECONDARY L. organs
ONLY IN BE CELLS!!!!! NOT T CELLS!!!
This refers to random point mutations that occur in the variable region of B
cells of an expanding clone in the germinal centers of the secondary
lymphoid organs.’
It occurs after antigenic stimulation of the B cells
This mutation allows for the creation of different variations of the antigen
binding site, and many of the mutations may result in a decline or even in a
loss of antigen binding. Therefore, the next and crucial step in the process of
affinity maturation is the selection of the useful, high-affinity B cells. Follicular
dendritic cells (FDCs) in the germinal centers display antigens, and the B cells
that bind these antigens with high affinity are selected to survive (see Figure
3 of the Lecture 6 notes). As antigen is cleared, smaller and smaller
amounts of antigen will be available to bind antibody. Due to the limited
availability of antigen, only those B cells with antibody mutated to have higher
affinity will be able to bind antigen and survive. B cells that do not bind
antigen die by apoptosis. Those B cells with antibody mutated to have lower
affinity for antigen will also die off since they will not be able to “compete” as
well for antigen.
Thus, within an expanding clone of B cells, B cells that have mutations that
result in a better-fitting antigen binding site have a selective advantage over
other members of the clone. Consequently, such B cells are more likely to
survive and proliferate, shifting the population of antibodies towards those with
higher affinity for antigen
CLASS (ISOTYPE) SWITCHING
only happens to antibody heavy chains
IN SEC LYMPH ORGANS
Class switching is dependent upon both antigenic stimulation and the presence of
factors released by T cells. These factors are referred to as cytokines, which
you may remember are soluble proteins secreted by variety of cells (including T
cells) that function as growth and/or differentiation factors. In the context of B
cell class switching, cytokines provide the signals necessary for B cells to
produce antibodies of different classes. Note that T cells can signal B cells to
produce specific classes of antibodies depending on specific characteristics of
the infection, such as what type of infection it is and where the infection is
occurring. Certain cytokines signal the B cell to produce certain antibody
classes.
antibody with a single antigen specificity to be produced in a variety of different
classes, thus allowing the antibody response to take on a number of different
effector functions. (Recall that the antibody class determines the “biological
activity”, or the specific effector function(s) of the antibody produced.)
It is also worth noting that, with class switching, each individual B cell that results
can produce only one antibody class. While individual mature B cells can
produce both IgM and IgD on their cell surfaces, after the process of class
switching, an individual B cell can only produce one of the so-called “switched
isotypes” (i.e. IgG or IgE or IgA), not a combination of them (e.g. both IgG and
IgA, both IgE and IgA, etc.)
**starts at DNA with help of DNA
