Properties and Functions of Antibodies Flashcards
B cells employ a range of different genetic approaches to modify antibody function. Match the function to the underlying genetic approach.
-Simultaneous production of IgM and IgD
-Class/isotype switching
-Affinity maturation
-Switch between membrane and soluble forms
- Permanent DNA rearrangement
- Somatic hypermutation
- Differential RNA processing
- Differential RNA processing
- Permanent DNA rearrangement
- Somatic Hypermutation
- Differential RNA processing
The switch between membrane and soluble forms of antibodies, and the production of IgM and IgD, are both controlled by differential RNA processing.
Antibody class or isotype switching is mediated by a permanent DNA arrangement. For example, to class-switch from IgM and IgD to production of IgG, the B cell deletes the IgM and IgD constant regions from the heavy chain gene locus. This brings the IgG constant region next to the rearranged VDJ segments (that encode the variable region that defines Ag specificity) resulting in the IgG constant region being joined to the variable region.
Affinity maturation is the overall process by which B cells modify their antibody antigen binding sites (variable region) to increase their affinity for their antigen. The VDJ segments encoding the variable region is modified through a process called somatic hypermutation, in which they undergo a much higher frequency of spontaneous mutations.
Which of the following statements about Ab class switching are true? Select all that apply.
-B cells can secrete IgA and IgG simultaneously
-IgM is the first Ab to be produced in large quantities
-Involves the light chain
-An IgA secreting B cell can never secrete IgG again
-Requires the enzyme AID (activation-induced deaminase)
-Changing the class of Ab alters its ability to bind to its Ag
-IgE can bind to all immune cells via its Fc region
-IgM is the first Ab to be produced in large quantities
-An IgA secreting B cell can never secrete IgG again
-Requires the enzyme AID (activation-induced deaminase)
IgM is the first antibody isotype to be produced in large quantities and the initial B cell response to a new antigen is largely IgM. Whilst B cells can produce IgM and IgD simultaneously (and are the only two Ab isotypes a B cell can produce at the same time), IgD is produced at much lower levels and is mainly membrane bound rather than secreted.
The class or isotype of an antibody is defined by the constant region of the heavy chain, and thus class switching involves changing the heavy chain constant region and does not involve modification of the light chain. The order in which antibody isotypes are expressed partly depends on their proximity to the VDJ variable region. IgM and IgD are proximal to the variable region and so these isotypes are produced first. The next closest is IgG, then IgE, followed by IgA. To class switch to IgA the B cell permanently deletes all the other intervening constant region sequences (encoding IgM, IgG, and IgE) from the heavy chain locus. This brings the IgA constant region directly next to the antibody variable region resulting in them being joined together. As the gene segment encoding IgG would have been permanently deleted during a switch to IgA this means that an IgA producing B cell will never be able to produce IgG again. Class switching requires the enzyme AID.
As class switching only modifies the constant region of the antibody, and does not affect the variable regions that form the antigen binding site, changing the antibody isotype does not change its ability to bind antigen, or its affinity for that Ag. As the heavy chain constant region defines effector function this allows antigen-specificity and effector function to be controlled independently.
Which of the following statements about affinity maturation are true? Select all that apply.
- Also occurs in T cells
- Results in B cells secreting higher affinity Ab
- Can result in self-reactive antibodies
- Only a minority of changes to the VDJ region result in a higher affinity Ab
- It is mediated by RAG proteins
- Involves spontaneous mutations in the VDJ region
- B cell survival during affinity maturation depends on membrane, not soluble, Ab.
- Does not require Ag to be present
- Involves a second rearrangement of VDJ genes
- IgM has the highest affinity for Ag
- Results in B cells secreting higher affinity Ab
- Can result in self-reactive antibodies
- Only a minority of changes to the VDJ region result in a higher affinity Ab
- Involves spontaneous mutations in the VDJ region
- B cell survival during affinity maturation depends on membrane, not soluble, Ab.
The underlying mechanism of affinity maturation is somatic hypermutation, which involves a high frequency of spontaneous mutations in the VDJ genes encoding Ab variable regions. The VDJ gene rearrangement itself only occurs once, and this requires RAG proteins. Somatic hypermutation requires the enzyme AID, the same as class switching.
Overall, affinity maturation results in the survival of B cells with Ab that have an increased affinity for Ag. However, as the mutations occur randomly only a minority will successfully increase the Ab’s affinity for Ag, and the majority of B cells undergoing somatic hypermutation die. During affinity maturation B cells have to be stimulated via Ag and membrane Ab (B cell receptor, BCR) to survive. B cells whose mutations prevent Ag binding will no longer be stimulated via their BCR and so will die. Those that still bind their Ag will have to compete for a limited Ag supply, meaning that only those with the highest affinity receptors for Ag will survive. Thus, Ag does have to be present.
As affinity maturation tends to take place after B cells have class switched away from the IgM isotype, B cells expressing IgM tend to have a lower affinity for Ag than B cells expressing IgG, IgA or IgE.
A dangerous outcome of affinity maturation is that the mutations in the Ag binding region could result in a pathogen-specific B cell becoming accidentally specific for a self-Ag instead, resulting in an autoimmune response. To prevent this happening B cells must ask permission from an activated T cell following somatic recombination. Activated T cells have already been given permission to respond by DC using signal 1 and signal 2 to show that the Ag the T cell recognises comes from a pathogen. To get T cell permission, B cells take up the Ag they recognise by internalisation of their BCR-Ag, break it up, and present peptide fragments to Th cells via MHC class 2. If an activated Th cell recognises the peptide then it is safe to assume that the Ag came from a pathogen. If a Th cell does not recognise the peptide then the mutations have likely caused the B cell to recognise a different Ag, potentially a self-Ag, but without Th cell permission the B cell will die. Because T cells act as a safeguard against B cells accidently becoming autoreactive it is important that TCR is not modified post-activation by DC, this ensures that T cells remain pathogen specific. Thus, the TCR does not undergo affinity maturation or somatic-hypermutation.
Match the Fc Receptors to the Ab class that they bind
FcγR
FcαR
RcεR
A. IgG
B. IgE
C. IgA
FcγR - IgG
FcαR - IgA
RcεR - IgE
The FcγR binds to the Fc region (heavy chain constant region) of IgG, FcαR binds to IgA, and FcεR binds to IgE.
By coordinating expression of different FcR on different innate cells the immune system can control which antibody isotype partners with which immune cell.
