preview:• also known as immunoglobulins (Ig)
• Ab are the targeted weapons of the adaptive immune system
• clear extracellular pathogens and neutralise toxins
• produced by activated, effector B lymphocytes (plasma cells)
• circulate in plasma and lymph and are found at mucosal surfac...
What are antibodies?
• also known as immunoglobulins (Ig)
• Ab are the targeted weapons of the adaptive immune system
• clear extracellular pathogens and neutralise toxins
• produced by activated, effector B lymphocytes (plasma cells)
• circulate in plasma and lymph and are found at mucosal surfaces
• specific – bind only a single antigen (Ag)
• the antibody repertoire is very large – 109-1016 unique molecules
• Antibodies are able to clear extracellular pathogens and neutralize toxins they are
produced by activated effector B lymphocytes.
• These cells differentiate into another cell type called plasma cells.
• An activated B cell expresses immunoglobulin on its surface whereas a plasma cell no
longer expresses an antibody on its surface and instead secretes it into the
extracellular space.
• These cells are important in mediating our immune response to pathogen.
• They circulate in the plasma and lymph and are found at most mucosal surfaces.
• Antibodies are very specific and any single antibody will only bind to a specific
antigen
B cells have surface antibodies
• B cells have surface antibodies, they can use these surface antibodies to encounter
with antigen. To recognize that antigen and to draw that antigen in to the cytoplasm
and process that antigen for presentation of its peptides onto MHC.
Plasma cells secrete antibodies
• When stimulated a B cell can give rise to antibody secreting plasma cells and these
cells then secrete antibody into the intracellular milieu. This is where the antibodies
can exert their own effector functions in a cell free manner.
Antibody structure.
• An antibody is made out of 4 polypeptide chains.
• 2 identical heavy chains which come from the same gene.
• 2 identical light chains they also arise from the same gene. ( the light chains can
either be lamda or capa).they are functionally identical to each other.
• Constant region- is the same for each antibody or class
• Variable region- this is the antigen binding site. This is where the variability is found.
• Hinge region- is found in the middle and those identical binding sites on each of the
arms of the Y shaped molecule can bind two antigen molecules at the same time.
• This means that each individual antibody molecule is bivalent.
Antibody dissection
• Papain digestion releases three fragments
• 2 Fab (Fragment antigen binding)
, • 1 Fc (Fragment crystallizable/constant)
• Fc region
• allows Ab to interact with other parts of the immune system
• needs to be constant (within an Ig class, e.g. IgE)
• You can use the enzyme papain to digest the antibody molecule into 3 fragments.
• This digestion leads to 2 FAB pieces, this is the FAB section of the antibody where the
protein is able to bind to the antigen. And you end up with one Fc domain which is
what allows the antibody to interact with other parts of the immune system. It is
constant within a class so all IgE Fc regions will be identical.
Hinge region gives flexibility
• Hinge region is important because it gives the antibody flexibility.
• So it allows the antibody to have some structural flexibility when dealing with
different epitopes found on different pathogens that come into the body.
Structure of human Ig classes
• Heavy chain change (constant regions) leads to different sub-classes with different
functions
• Light chain can be EITHER kappa or lambda
– no difference functionally in these
• All Ig classes have a consistent Y shaped molecule, with a constant region at the
bottom.
• And the antigen binding domains at the distal ends.
• The constant regions are slightly different between the different classes.
• These constant regions lead to subclasses with different functions because the
constant regions interact with different receptors on cells
• Antibodies are compact protein complexes made up of immunoglobulin (Ig)
domains.
Ig domains- light chain
• Parts of the light chain will be constant.
• There is a light chain constant domain whereas there is also a light chain variable
domain
Antigen binding- hypervariable (HV) regions.
• HV regions (red)
– complementarity-determining regions CDR
– flanked by framework regions (yellow)
– There are 3 HV regions in each V domain
• Come together at the ‘tip’ of the Ig molecule
• Since antibodies are bivalent and the antigen binding region is made of variable
regions of heavy and light chains, there are 12 CDR per antibody
• Critical in an antibody:
• Hypervariable regions within the variable domain.
• Hypervariable region is part of the antibody that actually physically interacts with the
antigen.
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