4.12 Adaptive immunity B cells Flashcards
Where to lymphocytes develop and circulate?
- Lymphocytes develop in the primary lymphoid organs and enter the blood circulation.
- They can leave the blood and circulate through the secondary lymphoid tissues (every 24 hrs for T cells), but not any other tissues
- They are in a quiescent state - “naïve”
What is tolerance of B and T cells?
T and B cells with receptors that bind “self” antigens are removed during differentiation in the primary lymphoid tissues (there are exceptions!) - TOLERANCE
What are antigens?
- Antigens are molecules which are recognised by receptors on lymphocytes, and elicit a specific immune response to that antigen
- Antigens can be proteins, CHO’s, lipids , nucleic acids
At what stages does clonal selection and clonal expansion occur?
How and where are T and B cells activated?
- Occurs in the secondary lymphoid organs.
- B cells are activated when free antigens drain via the lymphatics to the lymph nodes
- Antigen Presenting Cells (dendritic cells and macrophages) which have internalised antigen in the tissues migrate via the lymphatics to the lymph nodes and display degraded antigen peptides to T cells.
What are epitopes on antigens for B cells?
- The site where the antibody binds is called the epitope or determinant
- There can be more than one epitope recognised in any given antigen
- Protein determinants can be linear or discontinuous
- B cell receptors bind to conformational shapes
What are the two forms of Ig?
- Surface Ig (IgM) - embedded in the B cell membrane which acts as the B cell antigen receptor
- Secreted Ig - secreted by activated B cells (plasma cells)
- Each B cell expresses multiple copies of Ig (around 105/ cell) which bind the same antigen
What are the two distinct regions of antibodies?
- The constant region (Fc) can take one of 5 forms (isotypes)
- The variable region (Fab) can take one of an almost infinite variety of forms
Which part of the antibody binds the antigen?
What is the difference between antigen binding and antibody affinity?
- Antigen binding is the function of the F’ab region
- Antibody affinity is a measure of the strength of the bond
- between an antibodies binding site, and an antigen.
- Usually, the higher the affinity, the better the outcome of antigen- antibody binding
What are the five different isotypes of antibodies produced by B cells?
What are the different calsses of antibodies?
- IgM - on the membrane of naïve B cells, secreted into plasma as a pentamer
- IgG - high concentrations in plasma and tissue fluids
- IgA - high concentrations in mucosal secretions (often a dimer)
- IgE - low levels in plasma, tissue fluids, (bound to mast cells)
Considering the F’ab region what can antibody binding to antigen on a pathogen results in?
- Prevention of pathogen binding at mucosal surfaces (IgA)
- Neutralisation of toxins (IgG, IgM)
What does antibody binding to antigen result in on the F’c region?
- Triggering the classical pathway of the complement cascade (IgG, IgM)
- Opsonisation : the Fc region binds directly to Fc receptors (FcR) on phagocytes (IgG;).
- Transplacental immunity (IgG)
- Triggering degranulation of mast cells, eosinophils (IgE)
- Inducing the release of cytotoxic products from macrophages and NK cells (by binding FcR)
Which antibody isotype is released after antigen encounter?
- Remember, the antibody found on the surface of naïve B cells, and released after antigen encounter, is IgM
- Membrane bound Ig Is B cell antigen receptor
IgM
How does isotype switching occur?
- Occurs only after B cells have been stimulated by antigens to make IgM
- Involves irreversible DNA recombination events
- External signals (from T cells and the pathogen) determine the isotype produced
- Is accompanied by an increase in the affinity of the newly produced isotypes
How does affinity maturation occur?
- Antibodies produced after antigen binding to surface IgM “mature”
What are the genes for heavy chain to chose from in antibodies?
What are the genes for light chain to chose from in antibodies?
When does immunoglobulin gene rearrangements occur?
- Immunoglobulin gene rearrangements occur during the development of B cells from pluripotent stem cells in the bone marrow
- Heavy chain rearranges first, then light chain genes
- Rearrangement accompanied by rapid division and many cells die
- Many enzymes critical for this process
Which genes and enzymes control differentiation in the heavy and light chain gene rearrangements in B cells?
- Heavy and light chain gene rearrangements in B cells are the result of random somatic gene rearrangements (D-J, V-DJ, V-J)
- Differentiation controlled by genes (eg btk)
- Recombinations of V, D and J genes are facilitated by recombinase enzymes - recombinase activating genes (RAG’s), terminal deoxynucleotide transferases (TdT ), and exonucleases
- Defects in above genes result in a block in recombination and an absence of circulating B cells
What happens if immature B cells display auto-reactive antibodies?
- Before leaving the bone marrow, those that bind self molecules in the bone marrow are deleted – note that this process is not perfect!
- B cells that survive this process migrate to the circulation, where they now express IgM antibodies of a single specificity
How does isotype switching occur?
- Change the constant domains
- Recombine VDJ elements that are fixed but change switch sequence
- Does not switch of Fc recombination genes
When exactly does isotype switching occur?
- Occurs only after B cells have been stimulated by antigens
- Involves irreversible recombination events
- External signals (from T cells or the pathogen) determine the isotype produced
