Term
| How is B-cell mediated antigen responsiveness different from T-cell mediated responsiveness? |
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Definition
B cells can interact DIRECTLY with antigens while T cells require MHC presentation.
Remember, B-cells present antigens to CD4+, Th Cells, which release cytokines that stimulate B-cell-mediated production of Ab for a memory response. |
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Term
| How do receptors for antigens in the adaptive immune response differ from innate receptors such as TLRs, MHC and cytokine receptors? |
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Definition
SHORT-
Adaptive= Somatic Rearrangement/Allelic exclusion/Clonal Expansion
Innate= Germline encoded w/ co-dominant expression
1) They are derived from DNA changes specific to that particular cell, so called "somatic rearrangement"
2) Only 1 of 2 alleles is rearranged with "allelic exclusion" of the non-rearanged allele (different from co-dominance of MHC)
3) results in Clonal Expansion |
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Term
| What is the general structure of an antibody and what 2 primary forms can it exist in? |
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Definition
1) Monomer with 2 light chains and 2 heavy chains (LC kappa, LC lambda and HC are all on different chromosomes)
Each LC has 2 "Ig domains," 1 Variable (composed of V + J segments) and 1 Constant (2 of each for H2L2 monomer)
Each B-cell has 1 light chain isotype of LC-kappa or lambda, and both LCs in a given Ab have the same somatic rearangments
Each HC has at least 4 "Ig domains" (MW= 50kDa)
LC and HC paired in L2H2 dimer (150 KD per Ab)
2) Secreted or BCR forms in B-cells (mRNA splicing). |
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Term
| Explain how H2L2 Antibody monomers are digested and what this digestion produces. |
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Definition
1) Papain protease digests monomer into 2 antigen-binding fragments (Fabs) and 1 constant region (Fc)
2) Each Fab (variable sequence) binds 1 antigen (valence =1 )
3) Fc (COOH end) links innate and adaptive immune systems, but does NOT bind antigen.
Each Fc has a COOH end, a Complement fixing portion, and contacts receptors for Ig Fc. |
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Term
| How can you get both a BCR ad a secreted form of the Ig molecule from B-cells? |
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Definition
| Alternate RNA processing of single primary transcript |
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Term
| Explain the role of "recombinatorial associations" and "random pairing" in Fab/Antigen binding. |
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Definition
Combinatorial and Pairing diversity results in >1.8 million possible BCRs!
Remember, the Fab region of the protealized antibody is responsible for antibody and is highly variable in terms of AA sequence (variability in hyper-variable CDR regions, NOT framework regions of beta-sheets)
1) Recombinatorial Associations such as V1:D1:J1 or Vn:D3:J2 are enabled by different choices of which V, D, or J (heavy) of V:J (light) segments are made in individual B- cells.
2) Each of these pairing options would have a different AA sequence, so any HC can pair with any LC through "random pairing."
Bottom Line is that each B cell can encode a different BCR with different antigen binding capabilities contributed by H and L, despite coming from a clonal cell originally! |
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Term
| What is meant by "junctional diversity" in BCR rearrangements and how does it contribute to diversity? |
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Definition
SHORT- TdT adds random nucleotides at V;D and D:J joining sites of heavy chain, in addition to random addition/subtraction resulting from joining.
-Joining itself is "imprecise" (i.e. nucleotides are added/subtracted) and terminal deoxynucleotidyl transferase (TdT) can add random nucleotides (N-regions) at V:D and D:J joining to coding segments in heavy chain.
Note- "P nucleotides" and "primed" in the DNA |
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Term
| Explain the importance of CDRs/hv regions in sequence variation for the light chain of an antibody. |
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Definition
CDR1 and CDR2 exhibit germline variation in terms of VL's with hv regions, and NO junctional diversity.
CDR3 exhibits germline variation with many J's, with junctional diversity as well (since this is the "joining" site) |
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Term
| Explain the importance of CDRs/hv regions in sequence variation for the heavy chain of an antibody. |
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Definition
CDR1 and CDR2 have germline variation with many VHs with hv regions, but no junctional diversity
CDR3 has germline variation with MANY short Ds and Js, as well as LOTS of junctional diversity. |
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Term
| Explain how CDR/hv regions relate to BCR gene rearrangements (general description) |
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Definition
Remember, a CDR needs to "complement" an antibodies shape, because it is the region where contact is made with the antigen (strangely, all rearrangement occurs before this contact)
A CDR is a part of the variable (V) domain of an antibody (1 CDR1, 1 CDR2, and 1 CDR3 per each V domain so there are 12 CDRs for each Antibody molecule posessing 2 antigen-binding regions!) |
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Term
| What does it mean that pathogens have many "antigenic determinants"? |
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Definition
| They can be recognized by many different antibodies/T-cells |
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Term
| What are some general characteristics of an "Ig-like" domain |
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Definition
1) Disulfide bonds hold confirmation together
2) Dissociated regions (framework sections) composed mostly of B-sheets |
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Term
| What are the 4 "Cs" of the Fc region of an antibody? |
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Definition
1) Constant region fragment
2) COOH end of molecule
3) Complement fixing portion (for CR2 association in B-cell activation)
4) Contacts Fc receptors on cells |
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Term
| Where does the majority of HC and LC variable occur in an antibody molecule? |
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Definition
In the "hyper-variable" CDR regions of the Fab domains.
CDR1 and CDR2 are in V region
CDR3 is in J region (junctional diversity) |
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Term
| What are the 2 primary processes that contribute to BCR diversity in B-cells? |
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Definition
1) Re-combinatorial associations (DONT forget allelic exclusion) in CDRs of Fabs
2) Random pairing of HC and LC |
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Term
| What are the 2 sources of junctional diversity in CDR3s in J regions? |
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Definition
1) Imprecise joining (added/subtracted nucleotides)
2) Terminal deoxynucleotidyl transferase (Add random nucleotides)
** Can lead to AA substitutions, frameshift mutations and even premature stop codons** |
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Term
| What is the role of the RAG complex in junctional diversity? |
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Definition
This is a way of generating junctional diversity in the HEAVY CHAIN.
1) RAG binds and cleaves recombination signal sequences to yield DNA hairpins
2) RAG-mediated cleavage of hairpins generates palindromic "Primed" nucleotides
3) TdT adds N-nucleotidees to open D and J sites
4) Strands re-pair and unpaired nucleotides are removed. |
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Term
| What is the "enigma" of gene rearrangement in BCRs? |
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Definition
| All gene rearrangement occurs in the absence of an antigen, yet rearrangements DRIVES early development of B-cells (as well as T-cells) |
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