Antibodies Flashcards
Naive B cells
cells that have not met antigen, circulate from blood into peripheral lymphoid tissues (main site on antigen encounter)
tell me about the structure of lymph nodes
cortex and inner medula
- Cortex has an outer section of B cells organised into follicles
- a paracortical area of T cells and dendritic cells
- Germinal Centres of B cell proliferation form during an immune response
- Medulla consists of macrophages and antibody secreting B cells (Plasma cells)
- Antigen (in DC) enter through afferent lymphatics.
red pulp of the spleen
site of RBC destruction
white pulp of the spleen
White pulp is lymphoid
pathway through the spleen
- Blood carrying lymphocytes
and antigen enter from a trabecular artery into a central arteriole - they pass into a marginal sinus and exit through a trabecular vein
marginal sinus
surrounded by lymphocytes, and within it is the periarteriolar sheath (PALS), made up of T cells
B cell follicles and a B cell corona also form. Important - antigen enters from blood rather than lymph.
key features of adaptive immune response
antigen specificity, and memory the secondary (memory) response is faster, can produce more antibody, and does not prevent you from making a response to another antigen (multiple antigens -vaccination)
Antibodies can be expressed as
membrane bound (B cell receptor - BCR) or secreted forms. B cells express a single Ab specificity only
what are the functions of Ab cells
- firstly to bind the pathogen that elicited its production
- secondly to recruit other cells and molecules that will lead to clearance or destruction of the pathogen
Ab structure
- made of four polypeptide chains, two identical heavy (H) chains and two identical light chains (L)
- The H chains are disulfide bonded to each other, and each H chain is also disulfide bonded to a L chain
two types of L chain
lamba (!) and kappa (“). Any individual Ab has either ! or “, never a mixture of each. In human the ratio is 2:1 in favour of “.
Two identical H and L chains results in two identical binding sites.
Crystal structure of antibody shows a Y shape consisting of three globular domains
VL - variable light, CL - constant light, VH - variable heavy, CH1 - constant heavy 1
Proteolytic digestion reveals
functional domains of Ab
The binding site for antigen
is contained where
in the Fab region whilst The Fc region performs many of the functions of Ab, interacting with receptors etc
what can be used to label cells
Fab and F(ab)2 fragments
are very us in the lab without inducing the effects
of the Fc region, or in the case of Fab fragments, inducing receptor triggering or internalisation by crosslinking
Hypervariability in V domains
- Three hypervariable loops determine antigen specificity by forming a surface complimentary to the antigen.
- Final specificity is determined by a combination of loops from H and L chains, and not either alone.
Antigen binding to Ab
Antigens can bind in pockets or grooves, or on extended surfaces
antibody repertoire
The total number of antibody specificities available in an individual
humans its = 10’11
Germline Theory
suggested that a separate gene existed for each Ab. Not really the case.
Somatic Diversification theory
proposed that repertoire is generated from a limited number of V region genes that undergo alteration. Essentially correct.
Chromosomal Rearrangement
The sequence of a V region is generated by the somatic recombination of separate gene segments
Junctional Diversity
N-nucleotides, (non template-encoded) are added by terminal deoxynulceotidyl transferase (TdT).
After upto 20 nucleotides are added, pairing is attempted, followed by trimming off nonmatching bases, filling gaps and ligation to complete.
• Ab diversity is created by
- 1 -rearranging multiple gene segments
- 2 - junctional diversity
- 3 - different combinations of H and L chains
- All the above happen during B cell development, long before it ever sees antigen
Somatic Hypermutation
occurs after B cells have become antigen activated in the lymph node.
affinity maturation.
During the course of an immune response 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
Five classes of Ab
IgM IgG IgD IgA IgE
Cell surface IgM
All B cells initially express a membrane form of IgM (some also IgD). After antigen stimulation they can secrete a soluble version of the same IgM made by alternative mRNA processing
what supplies functional diversity
Cells also undergo isotype switching, where the IgM Heavy chain is swapped for IgG etc
Antibody multimers
IgM can be secreted as pentamers, IgA as dimers. Both involve an additional J chain in this process.
Selective distribution of Ab isotypes in body.
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.
The fetus receives IgG by transplacental transfer.
IgE is found mostly near to epithelial Surfaces, especially gut, lungs and skin.
The brain is normally free from Ab.
high levels of Ab-Ag
high levels of Ab-Ag complexes are prevalent in some autoimmune disease, major cause of glomerulonephritis
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.
