Antibodies Flashcards
Which cells produce lymphocytes ?
Beta Lymphocytes
Explain the journey of antibodies, starting at its origin.
Arise from stem cells in bone marrow –> differentiate in the central lymphoid, organs B cells in bone marrow, T cells in thymus –> migrate in bloodstream (still naive) to peripheral secondary lymphoid organs (yellow), lymph nodes, spleen, gut-associated lymphoid tissue (GALT), Peyers patches, tonsils, appendix
Explain what is meant by naive beta cells ?
Beta cells that have not met antigen
What is the main site of antigen encounter for naive beta cells ?
Peripheral lymphoid tissues
Where do beta cells go following drainage of tissue fluid through the lymph nodes and into the thoracic duct ?
Recirculating B cells enter back into the bloodstream by the thoracic duct and joins lymph node in arterial blood
Describe the organisation of lymph nodes.
Cortex and inner medulla
CORTEX
Has an outer section of B cells organised into follicles, a paracortical area of T cells, dendritic cells, and Germinal Centres of B cell proliferation (form during an immune response)
MEDULLA
Contains macrophages and antibody secreting B cells (Plasma cells)
AFFERENT LYMPHATICS
Brings in antigen
What are the secondary lymphoid organs ?
Spleen, lymph nodes, Peyer’s patches, tonsils
What are the primary lymphoid organs ?
Red bone marrow, thymus
How do antigens get into the spleen ? into the lymph nodes ?
From blood rather than lymph.
Through afferent lymphatic vessel
Describe the organisation of the spleen.
RED PULP
Site of RBS destruction
WHITE PULP (lymphoid)
- Blood carrying lymphocytes and antigen enter from a trabecular artery into a central arteriole
- Then pass into a marginal sinus and exit through a trabecular vein
- Marginal sinus surrounded by lymphocytes
- Within marginal sinus it is the periarteriolar sheath (PALS), made up of T cells
- B cell follicles and a B cell corona also form
Describe the course of antibody production upon meeting an antigen for the first, and second time.
1) Primary Response
- Take a few days (because adaptive immunity)
- Sudden increase, then progressive decrease of antibody concentration (because plasma cells are short lived)
2) Secondary Response
- Faster and larger (more antibodies) response
- Because some B cells made in primary response do not die but turn themselves into memory B cells (go back into lymph nodes and sit there if necessary
- Secondary response to one antigen does not prevent you from making a response to another antigen )
Are antibodies membrane bound ?
Can be expressed as membrane bound (B cell receptor - BCR) or secreted forms.
How specific are B cells in producing antibodies ?
B cells express a single Ab specificity only
What are the main functions of antibodies ?
1) Opsonisation, enhances phagoyctosis
2) Neutralisation (of viruses, toxins) by binding “to antigens (on virus, or toxin) and block their attachment sites”
3) Complement activation, activate complement cascade
4) Agglutination
Describe the structure of antibodies.
- four polypeptide chains
- two identical heavy (H) chains
- two identical light chains (L)
- H chains are disulfide bonded to each other, and each H chain is also disulfide bonded to a L chain
- Two L chain exist, lamba (λ) and kappa (ϰ). Any individual Ab has either one or the other, never a mix of the two.
- N termini bind pathogens whilst C termini binds cell and activates immune response
What is the ratio of λ L chains to ϰ L chains in the body ?
2:1 in favour of ϰ
What is the variable and what is the constant part of an antibody ?
Variable - Antigen-binding part
Constant - Activating part
How may antibodies be digested ? What is the possible use of it ?
Through proteolytic digestion (using papain, or pepsin)
Papain cuts it in two fragments: Fab (contains antigen binding site) and Fc (interacts with receptors)
Pepsin actually leaves the two Fabs stuck together
Fab and F(ab)2 fragments can be used to label cells, or induce receptor triggering or internalisation by crosslinking, without inducing the effects of the Fc region (e.g. not activate complement cascade)
Where are the hypervariable parts of antibodies found (CDR regions)?
Antigen-binding site
Is the final specificity of the antigen-binding site determined by the L, H chain or both ?
Final specificity is determined by a combination of loops from H and L chains
What is the shape of the antigen-binding site ?
Pockets or grooves, or on extended surfaces.
Describe the processes which lead to the formation of the human antibody repertoire (i.e. generation of antibody diversity). At which stage of B cell development does each of these occur ?
DURING B CELL DEVELOPMENT
- Sequence of a V region is generated by the somatic recombination of separate gene segments - Chromosomal Rearrangement (Somatic Diversification theory)
- Genome contains multiple V, J and D genes
- Junctional Diversity: N-nucleotides, (non template-encoded) are added by terminal deoxynulceotidyl transferase (TdT). After up to 20 nucleotides are added, pairing is attempted, followed by trimming off non-matching bases, filling gaps and ligation to complete.
AFTER B CELL HAVE BECOME ANTIGEN ACTIVATED IN LYMPH NODE
- Somatic Hypermutation: mutations accumulate in the V regions of the H and L chain genes. Some of these will bind antigen better and these cells are selected to expand and secrete antibody. This is called affinity maturation
What is the total number of antibody specificities present in a human ?
10^11
What are the main classes of antibodies ? What are they defined by ?
IgG, IgM, IgE, IgA, IgD
Defined by heavy chain
What are some of the differentiating characteristics between different antibodies ?
- Concentration in the serum (e.g. IgG high concentration in the serum whilst IgE low concentration due to being mainly bound to mast cells rather than free)
- Heavy chain
- Half-life
- What it does best (e.g. opsonisation, activation of complement system)
Is IgM present in the cell surface or in the cell ?
- B cells initially express a membrane form of IgM
- After antigen stimulation they can secrete a soluble version of the same IgM (alternative mRNA processing)
Give an example of a mechanism supplying functional diversity of antibodies.
Cells undergo isotype switching, where the IgM Heavy chain is swapped for IgG etc
Identify the antibody multimers. How are these linked ?
IgM- pentameres
IgA- dimers
Both involve an additional J chain in this process.
Where in the body are the different antibodies found ?
- IgG and IgM predominate in plasma
- IgG and monomeric IgA are the main isotypes in extracellular fluid
- Dimeric IgA predominates in secretions across epithelia, including breast milk
- Fetus receives IgG by transplacental transfer.
- IgE found mostly near epithelial surfaces, especially gut, lungs and skin
Which antibodies are present in the brain ?
The brain is normally free from Ab.
How does mast cell degranulation occur ?
Antigen (from allergen) cross-links with IgE, causing release of granule contents
What are antigen immune complexes ? How does their number evolve over time ?
Complexes formed by antibody + antigen
Initially not very many, then equivalence point, then too many
Eventually cleared away using phagocytosis
How may high levels of Ab-Ag complexes be problematic ?
Prevalent in some autoimmune disease (Immune complex diseases):
- Glomerulonephritis (deposition of said immune complexes in kidneys)
- Vasculitis
- May also affect skin
How are monoclonal antibodies developed ?
- Take cancer cell (eg myeloma), mix it with B cells, and fuse them together (through chemicals, electrical currents)
- Although 99.9% of cells die as part of process, those that survive are hybridomas, with characteristics of myeloma (grows forever), as well as characteristics of B cell (secreting antibody)
- Select antibody-secreting hybridoma you want (secreting one antibody) and you can grow that as much as you want
Identify specific monoclonal antibodies, and state their use.
INFLIXIMAB
- Anti-Tumour Necrosis Factor (inflammatory mediator)
- Used in Rheumatoid Arthritis, Ankylosing Spondylitis, psoriasis, inflammatory bowel diseases.
HERCEPTIN
- Anti HER2 (human epidermal growth factor receptor 2)
- Can block growth and lead to destruction of breast tumour cells that express high levels of HER2
GLEEVAC
- Anti-tyrosine kinase
- Effective against Chronic Myeloid Leukemia
Does everyone respond to monoclonal antibodies ?
No
