Antibodies Flashcards
What is an antibody?
A protein that is produced in response to a foreign molecule (antigen), and has the property of binding specifically to that antigen
What are immunoglobins?
A large family of soluble glycoproteins. Antibodies form this class of proteins
What produces antibodies?
B lymphocytes
What happens after antibodies bind to antigens
- complement activation
- opsonisation
- cell activation via antibody-binding receptors (fc receptors)
What is this showing
The band in monoclonal expansion column is indicative of myeloma (cancer that develops from the bone).
Antibodies are in the gamma immunoglobin category- increased gamma globin gives a strong antibody response.
When there is a diffuse band, there are is large variety of antibodies
Describe the anatomy of an antibody
It is a symmetrical molecule with 2 light chains (outside) and 2 heavy chains (inside).
Both light chains are identical and both heavy chains are identical. The N and C symbolise the amino and carboxyl ends of the polypeptides.
The chains are held together by disulphide bridges
What did rodney porter do (experiment)?
He digested gamma globulins (antibodies) with purified papain.
The gamma globulins produced 3 fragments in equal amounts.
The 2 fragments which bind to the antigen =Fab
The fragement that didn’t formed protein crystals- Fc
If you digest the fab with mercaptoethanol (breaks disulphide bonds), there are equal amounts of heavy and ligh chains.
Flexible- hinges to bend
Why is it important that the antibodies are flexible?
To be able to accomodate widely and closely spaced surface determinants. The flexibility allows both antibody fabs to bind to antigens.
Different regions in the light and heavy chains.
They can be divided into variable and constant regions.
The variable part is where the antigen binds to so in the picture it will be towards the LHS. The amino acid sequence of the variable region must vary to give the antibody different specifities.
Fc does not have to be variable because it does not bind to the antigens.
Disulphide bridges in the antibody
the light and heavy chains are held together by disulphide bonds
BUT
There are internal intrachain disulphide bonds- these are immunoglobin domains. Lots of other proteins have immunoglobin domains
The Fc part changes conformation when the antigen is bound and can perform effector functions like activating complement.
Antibodies are cross-reactive. What does that mean?
antibodies are highly specific but they can react with other molecules.
What does this picture show?
It shows that variability in the amino acid sequence is not evenly distributed.
There are 3 regions which are hypervariable. They are called the complimentarity determining regions (CDR)
What are complimentarity determining regions?
Loops of amino acids that are a part of the protein that is binding to the antigen ( and determines the specifities).
they are located at the ends of the protein and they interact with the antigen.
Fc region has barrell shaped beta-pleated sheet, which is held together by internal disulphide bonds
Where are the CDRs located?
They line up at the ends of the variable domains.
Fc has barrell shaped beta pleated sheet.
Between an antibody and antigen, there are 20 different interactions binding them together.
What are the forces between antibodies and antigens?
- Hydrogen bonds
- ionic bonds
- Hydrophobic interactions
- van de waals interactions
The forces are non-covalent. Together, they are very strong
What is antibody affinity?
The strength of the total non-covalent interactions between a single antigen binding site and single epitope on antigen.
What is antibody avidity?
The overall strength of multiple interactions between an antibody with multiple binding sites and a complex antigen with multiple epitopes.
Antigens do not have just one binding site, they have at least two- the more binding sites that are bound to antigens, the higher the avidity
What are the different classes of antibodies? And where do they differ?
Different classes of antibodies differ in the constant regions of their heavy chains. There are 5 different classes of antibodies, A,D,E,G,M.
There are different genes for the different heavy chains in the different classes- look at the table (greek letters match the Ig letters).
The light chain can be kappa/ lambda- only one type per antibody because they are symmetrical- so either both kappa or both lambda
IgG and IgA have subsets- what are they?
There are 4 IgG subclasses- 1,2,3,4
Two IgA subclasses- 1 and 2
Describe the IgG antibody
Gamma for the heavy chain
It is the most abundant immunoglobin and has 4 subsets. Occurs as a monomer.
The variation between the 4 is located in the hinge region and effector function domains.
Actively transported across the placenta (passive immunity)- important for baby protection in the first few months but will fade out.
Major activator of the classical pathway for complement. (IgG1 and IgG3)
What are the properties of the 4 subsets of the IgG?
Why do we have subclasses?
There is variation in the hinge region and in the number of disulphide bonds.
Each subset has different effector functions and they are numbered 1-4 based on abundance. 1 being the most abundant.
The differ in their ability to activate complement
Describe IgA antibodies
alpha heavy chain
second most abundant immunoglobin after IgG.
Occurs as a monomer (blood) and as a dimer (secretions)
Major secretory immunoglobulin
It protects mucosal surfaces from bacteria, viruses and protozoa
How are IgA formed?
There is a process to form the dimeric IgA from below the epithelium layer into the lumen:
- the IgA is produced by a plasma cell
- The dimeric IgA binds to poly-Ig receptor on the basolateral membrane of the epithelial cell
- This triggers endocytosis
- The poly-Ig receptor is cleaved which just leaves the secretory component attached to the dimer IgA
- It is then secreted- the secretory component is derived from the poly-Ig receptor. The poly-Ig receptor contains a secretory component
Describe IgM antibody
mew heavy chain- large pentameric molecule. 5 monomers joined by J chain (10x Fab)
The antibody is mainly confined to the blood (80%) exposure. the first Ig synthesised after exposure to antigen (primary response). Multiple binding sites compensate for low affinity.
There are multiple binding sites which compensate for low affinity (high avidity)- efficient agglutination and activates complement. The whole structure is held together by disulphide bonds.
Describe IgE antibodies
epsilon heavy chain
In low concentration in the blood- it is produced in response to parasitic infections and in allergic diseases (IgE- allergeeeeee)
It binds to high affinity Fc receptors of mast cells and basophils. Fc sits on mast cell and Fabs grab the allergen. IgE coats mast cells and binds to Fc receptor on mast cell. The binding of the specific allergen causes a HUGE histamine release and mediate allergic reactions.
It is cross linking because it triggers antigens, activates mast cells and causes histamine release.
