Antibodies and B lymphocytes Flashcards
Immunoglobulin molecular structure: explain with the aid of a simple diagram the outline structure and function of the immunoglobulin molecule, identifying the antigen-binding site (Fab) and Fc portions of the molecule Antigen binding: explain how antibodies bind to antigen, and the consequences of such antigen binding B lymphocyte origin: explain the origin and maturation of B lymphocytes, including the principle of immunoglobulin gene rearrangement B lymphocyte activation: explain the proce
What is an antibody?
Protein that is produced in response to foreign antigen, specifically binds to antigens.
What are immunoglobulins – in relation to antibodies?
Form the class of proteins known as immunoglobulins and are a large family of soluble glycoproteins.
What cell produces antibodies?
B lymphocyte cells.
What type of immune response is associated with antibodies?
Adaptive immune response.
How many types of antibody are there in an individual?
More than 10^7.
What are secondary effector functions of an antibody? (x3)
Occurs after binding to the antigen. Functions: complement activation (the complement system is in ‘Organisation of the immune system’), opsonisation (promotion of phagocytosis), cell activation via specific antibody-binding receptors (Fc receptors).
What are Fc receptors in the context of an antibody’s secondary functions?
Fc receptors are found on the cell surface of immune cells like phagocytes and lymphocytes and allow cells to bind to antibodies that are attached to antigens of pathogens. Activating the immune cell.
What is the structure of an antibody?
N = amino-terminal ends. C = carboxyl terminal ends.
What are the Fab and Fc fragments of an immunoglobulin molecule?
Fab fragments of the antibody bind to antigen. Fc fragments bind to PHAGOCYTES – the region that phagocytes have receptors for.
What is the nature of the flexibility of antibody molecules?
The hinge region allows for flexible binding to antigens, so Fab regions can move to bind more widely spaced surface proteins.
Where are the constant and variable regions found on the immunoglobulin molecule?
Look at photo.
What is the importance of the variable regions of the immunoglobulin molecule?
Allows for specificity of the antibody to an antigen. The variable region is also the Fab region (since this is the antigen-binding region). The LOWER region of the constant region is the Fc portion.
What are the two types of disulphide bridge?
Inter and intra. Intra are found within the chains. Inter are found between the chains.
What are hypervariable regions of antibodies? How many in an antibody?
Within the variable domains of an antibody, there are three HYPERVARIABLE regions – called complementarity determining regions (CDR’s). These are the parts of the antibody that interact and bind with the ANTIGENS.
Where are the CDRs found on an antibody?
Found at the loops of the variable domains. Indicated by the annotation below.
What is the secondary structure of antibodies?
Made up of beta-pleated sheets, that loop. The beta pleated sheets are held by intra-disulphide bonds. Below indicated half of an antibody.
What forces are involved in the interaction between an antigen and antibody? What is the nature?
The forces ARE NOT COVALENT. Individually the interactions are therefore WEAK. So, many interactions – in reality – are involved in the bonding of antigen and antibody to compensate. Forces include hydrogen, ionic, hydrophobic, van der Waals interactions.
What two terms are used to describe antibody-antigen binding?
Antibody affinity and antibody avidity.
What is antibody affinity?
Affinity: Defined as the strength of the total noncovalent interactions between a SINGLE antigen binding site on the antibody and a SINGLE epitope on the antigen.
What is antibody avidity?
Defined as the overall strength of MULTIPLE interactions between an antibody with MULTIPLE binding sites and a complex antigen with MULTIPLE epitopes.
Which out of affinity and avidity is best for describing antibody binding capacity?
AVIDITY! Because antibodies have more than one antigen binding sites, and avidity describes strength of interaction for all binding sites.
What is antibody cross-reactivity?
Antibody elicited in response to one antigen can sometimes recognise different, similar antigen e.g. cowpox vaccination produces antibodies for smallpox.
What characteristics of the immunoglobulin molecule separates antibodies into classes?
Differences in the constant region of heavy chains.
What are the different classes of antibody? Light chain?
IgG, IgA, IgM, IgD, IgE (gamma, alpha, upsilon, sigma, epsilon). Each have either kappa (K) or lambda (up-side-down ‘y’) light chains.
How are subclasses notated for each antibody?
A number is added to the end e.g. IgG1, IgG2, IgG3, IgG4.
What is IgG? Common? Variability? Functions? (x2) Where? (x2)
Type of antibody with a gamma heavy chain, and the most abundant. Variability between subgroups usually in the hinge region. Actively transported over the placenta to give foetus/baby passive immunity for first few months of life. Major activator of classical complement pathway. Found mainly in blood and extracellular fluid.
What is IgA: Common? Where? (x2) How does it occur? (x2) Function?
Second most abundant. Found in epithelia and breast milk. Occurs as monomer in blood and dimer in secretions. Binds to basolateral surface of epithelial cells, where it enters the epithelial cell by endocytosis. It is then cleaved, and the remaining IgA forms a dimer secretory IgA with a longer half-life to neutralise pathogens in lumen upon release.
What is IgM: Structure? Where are they found? Functions? (x3)
Forms pentamers of basic structure: with 5 monomers joined by J chains (multiple binding sites compensate for low affinity). Mainly found in the blood. First Ig synthesised after exposure; efficient for agglutination (makes sense given multiple binding sites) and activating complement.
What is IgD: Where? (x2) Functions? (x2)
Extremely low serum (blood) concentrations; surface IgD expressed in early B cell development. Involved in B cell development and activation.
What is IgE: Where? Functions? (x2) Mechanisms? (x2)
Present at low levels. Produced against parasites and in allergy. MECHANISMS: Binds to high affinity Fc receptors of mast cells and basophils. Cross-linking by antigen triggers mast cell activation and histamine release (hence allergic reaction).
SUMMARY PHOTO OVERLEAF:
Be familiar…
What is the structure of a B-cell receptor?
It is a transmembrane protein complex composed of monomeric IMMUNOGLOBULIN (mIg) and di-sulphate linked heterodimers of IgAlpha and IgBeta on either side. Antibody stuck in membrane has a cytoplasmic tail that’s too short for signalling. Cytoplasmic tails of IgAlpha and IgBeta contain ITAM domains: they are long enough to allow for signalling into the cell.
How are BCR Immunoglobulin light chains expressed in DNA: from germline DNA to light chain as a protein?
- GERMLINE DNA contains 30-40 variable (V) regions, followed by 5 joining (J) regions and a constant region. This is found in IMMATURE B CELLS. During the joining of gene segments, the unused DNA is looped out and removed (see bottom part of photo – by VJ recombinase). 2. As B cell develops, one V region and two J regions randomly selected alongside the constant region. Recombination is imprecise, and parts of the V and J regions are chopped off slightly so that they can join together well – this also brings about more variation. 3. Alternative splicing patterns produce mature mRNA that has ONE V, J and C region – this is then translated to the actual light chain by ribosomes.
How are BCR Immunoglobulin heavy chains expressed by DNA: from germline DNA to protein?
- In germline DNA, there are around 65 variable regions (V), 27 Diversity (D) regions, followed by 6 J regions and 9 constant regions. During the joining of gene segments, the unused DNA is looped out and removed by VDJ recombinase. Recombination, remember, is imprecise = more variation. 2. Mature B cells have one V, one D, two J and ALL constant genes. 3. Primary transcript has VDJJ and constant regions for type of antigen present. 4. Alternative splicing produces heavy chain with one V, D, J and C: further produces diversity in heavy chains.
What are the three main differences between synthesis of heavy and light chains from germline DNA for the BCR?
In heavy chain synthesis, there is a D region, constant regions are functional, and constant region is synthesised by alternative splicing.
Incorporating what we know about B cell receptor synthesis, what is the OVERALL process of B cell maturation?
Stem cell precursors start in the bone marrow! • VDJ rearrangement of the HEAVY CHAIN GENES occurs in early pro-B cell and late pro-b cell. • VJ rearrangement of the LIGHT CHAIN GENES occurs in small pre-B cell. • Selection for self-tolerance (self-tolerance = failure to attack body’s own proteins) occurs during maturation. Those that are self-reactive are deleted by apoptosis.
What happens after there is BCR-antigen binding? (x1 immediate action and x3 possible routes.)
It holds out antigen ready for activation by CD4 T cell, and goes to the lymph node. It then does one of the following: 1. AFFINITY MATURATION: B cell becomes a germinal centre B cell (in the lymph node – refer to Immunology: Organisation of the Immune System), divides rapidly and mutates. Purpose: to improve the antibody response and create better B cells. B cells that enter this pathway can turn from germinal centre cells into memory cells and plasma cells (like the below pathways). 2. IMMUNOLOGICAL MEMORY: directly become memory B cells. 3. IMMUNOLOGICAL ACTION: directly become plasma cells – entering circulation where they produce antibodies – so have many endoplasmic reticulum and ribosomes. Really pay attention to the photo! – shows how affinity maturation can put B cells in the two other pathways too.
Thymus-dependent antigen pathway of B cell activation: why do cytokines only affect constant region of heavy chain, and not other genes?
If you look at the life-cycle of a B cell, the primary transcript RNA contains an unarranged constant region with exons still (in the heavy chain). SO, for mature B cells, cytokines can lead to different exons being translated for different constant regions. The constant region in the light chain is unaffected because it is only made up of one region, so cannot be rearranged.
Describe the antibody production in primary and secondary immune responses.
IgG and IgM!
