The Structure and Function of Antibodies Flashcards
What is an antibody?
The secreted form of the B cell’s B cell receptor
What mediates recognition of an antigen?
The recognition of antigen is mediated by the two antigen binding sites of the variable regions.
What determines the antibody’s biological role in the immune response?
The constant region of the heavy chain
How many polypeptide chains do all immunoglobulin molecules consist of?
Four polypeptide chains: two identical heavy (H) chains and two identical light (L) chains, held together by disulfide bonds.
How many types of L and H chains are there?
There are two types of L chains and five types of H chain.
The chains have Greek letter names.
What are the two types of L chain?
kappa (κ) chain and lambda (λ) chain
What are the five types of H chain?
the gamma (γ), mu (μ), delta (δ), epsilon (ε), and alpha (α) chains
What does the type of H chain define?
The type of H chain defines the immunoglobulin class or isotype.
The immunoglobulin may have a κ chain or λ chain L chain, but never both.
For example, both γ2λ2 and γ2κ2 antibodies are IgG antibodies because they have the same γ heavy chain even though they have different light chains.
What regions do all antibodies have?
Variable
Hypervariable
Constant
How many identical H and L chains does a single antibody molecule have?
A single antibody molecule has two identical H and two identical L chains.
Thus, a single antibody molecule has two identical antigen-binding sites.
Another way of saying this is that antibodies are bivalent.
What are the variable regions of antibodies?
The N-terminal portions of both the H and L chains are different for different antibodies and are therefore referred to as variable (V) regions
What are the hypervariable regions of the immunoglobulins?
Within the variable regions of the immunoglobulins, there are three short stretches of amino acids that vary the most from one antibody to another.
These are called the hypervariable (HV) regions.
The H and L chain V regions (VH and VL) combine to form the antigen-binding region of the antibody.
The hypervariable regions (HVH and HVL) contribute significantly to the antigen-binding site and are the part of the antibody that contacts the antigen.
The rest of the H and L chains are the constant (C) region.
Within a given species of mammals, the amino acid sequences of the constant regions are identical for all antibodies of that particular isotype.
Compare the C region of the H chain and L chain.
Each H chain (μ, δ, γ ε, α) has a different amino acid sequence in its C region. These determine the function of the antibody (how it helps eliminate the antigen). Different isotypes of antibodies have different roles in the immune system.
The C regions of the L chains (κ and λ) have different amino acid sequences but this doesn’t influence the function of the antibodies.
Describe the significance of the difference in the amino acid sequences of the C regions of the heavy chains between species.
It can cause problems in a clinical setting when a human patient needs to be transfused with antibody from a non-human species.
Immunologists can also exploit these differences to generate reagents for detecting and quantifying antibody.
For example, it is possible to generate an antibody in a goat that recognizes the heavy chain C region of the human antibody (because, in a goat, the human C region is recognized as a foreign protein).
These antibodies are called “goat anti-human Ig” antibodies because it is a goat antibody that recognizes the constant region of a human antibody.
What is the complementarity determining region?
Hypervariable regions are also called “complementarity determining region” or CDR.
What is the framework of an immunoglobulin?
The variable regions are sometimes called “framework” - while there is some variability in the amino acid sequences of different antibodies, there is also some conserved features.
These conserved features are important in the overall folding of the protein.
What part of the antibody makes contact with the antigen?
The hypervariable region shows extreme amino acid sequence variability - this is the part of the antibody that makes contact with the antigen.
In the three-dimensional structure, the hypervariable regions would be at the “tips” of the antibody.
What determines the specificity of the antibody?
The antigen-binding site is a three-dimensional pocket.
The side chains of the amino acids that make up the H and L chain hypervariable regions determine the shape of the antigen-binding site.
The shape of the antigen-binding site determines the specificity of the antibody (i.e., which epitopes it will bind).
What is an epitope?
A single antibody molecule binds to only to a small portion of the antigen molecule called the epitope or antigenic determinant.
An epitope has a unique three-dimensional structure.
The antigen-binding site of the antibody has a shape that is complementary to that of the epitope.
The epitope on the antigen fits into the antibody’s antigen-binding site in the same way a key fits into a lock.
How many epitopes does a single antigen have?
A single antigen can have many epitopes.
For a protein antigen, an epitope may be only 4–5 amino acids long (the average protein is 200–1000 amino acids long).
There may be many copies of the same epitope, different epitopes, or both.
Epitopes on the surface of the antigen are accessible to the antibody.
Therefore, many antibodies, each with different specificities, can bind to their particular epitope on one antigen particle.
Different antibodies have antigen-binding sites with different shapes, and will therefore bind epitopes with different three-dimensional structures.
It is possible for two different antigens to have an epitope in common.
True or false?
True
What is the chemical nature of an epitope?
The chemical nature of an epitope can be carbohydrate, lipid, amino acids, nucleic acid etc.
What is required for strong binding between the antibody and the epitope?
The antibody will only bind epitopes with a complementary shape (remember the lock and key analogy).
Close contact between the surface of epitope and surface of antigen-binding site is required.
A poor fit would suggest there is weak binding or no binding.
Close contact allows the formation of hydrogen bonds as well as ionic and hydrophobic interactions between the epitope and the antigen-binding site.
How many epitopes will a single antibody bind?
Only one or a few similarly shaped epitopes.
What are the main types of secreted antibodies?
IgM, IgG, and IgA
People with allergies also make lots of IgE.
Where is IgD found?
IgD does not seem to be secreted; it is only found on the cell surface of naïve B cells.
Why can antibodies of different isotypes recognize the same antigen?
Antibodies of different isotypes (also known as classes) can recognize the same antigen because they have the same V region (only their CH regions are different).
What is secreted IgM?
Secreted IgM is a pentamer consisting of 5 IgM monomers that are joined to each other by disulfide bonds.
Two of the IgM monomers are connected by a polypeptide called a J chain or Joining chain.
Describe the role of secreted IgM.
The antigen-binding sites of IgM antibodies often have low binding affinity.
However, pentameric IgM has high avidity (= total binding strength) because it has 10 binding sites instead of 2.
The more binding sites present, the greater the total binding strength.
The amino acid sequence in the μ chain constant region binds very effectively to complement proteins.
Bacteria that are coated with IgM will be killed when the complement is activated.
IgM can also neutralize pathogen.
Where is IgG found?
In the blood
IgG is found in the blood and can incapacitate pathogens by neutralizing.
It also helps to kill bacteria. Describe 2 ways it can do this.
- Complement proteins kill bacteria coated with IgG. An amino acid sequence in the γ chain constant region binds very effectively to complement proteins.
- Phagocytes ingest bacteria coated with IgG antibodies more readily than bacteria without IgG. An amino acid sequence in the γ chain constant region facilitates the binding of IgG to specific receptors on phagocytic cells.
IgG antibodies from the mother can also cross the placenta and provide protection for the fetus.
Describe the different isotypes (classes) of secreted immunoglobulin.
The B cell receptor is formed by the membrane bound version of monomeric form.
IgA and IgM are polymers connected by the J chain.
IgA as an additional protein called the secretory piece which is used to translocate it across epithelial layers and into mucosal secretions.
IgD is not thought to be secreted.
What is immunoglobulin class switching?
One of the important aspects of an antibody response.
In class switching, the VH region the same, but the B cell uses a different CH region. The L chain remains the same.
Thus, the class and function of Ig is different, but the antigen specificity remains the same.
How may B cells be induced to switch to a different class (isotype)?
Activated T helper cells may then induce some of the B cells to switch to a different class (isotype) of Ig by providing them with specific cytokines.
What is an important method for tailoring the adaptive response to best match the antibody to the pathogen?
Class switching.
The VH region (represent by X) remains the same - it is simply placed next to a different CH gene segment.
In this diagram, the B cell switches from matching the IgM form of the BCR to the IgG form of the BCR.
What is antibody neutralization?
Antibodies prevent viruses and toxins from binding to and entering cells (neutralization).
What is antibody opsonization?
Antibodies coat virus and bacteria, and improve the efficiency of phagocytosis by neutrophils and macrophages (opsonization).
What is antibody complement activation?
Antibodies allow more types of bacteria to be killed by complement proteins (complement activation).
Describe complement activation by antibodies.
Although complement proteins are regarded as part of innate immunity, they do work with IgM and IgG antibodies generated during an adaptive immune response.
- After IgM or IgG has bound to the surface of a pathogen, the early complement proteins are recruited, resulting in the formation of a C3 convertase.
- The C3 convertase cleaves C3 into C3a and C3b.
- The binding of C3b to the pathogen surface recruits additional proteins to form the C5 convertase, which cleaves C5 into C5a and C5b.
- C5b is also deposited on the pathogen surface and initiates the assembly of late proteins to form the membrane attack complex (MAC).
What plays are a large role in antigen binding?
The antigen binding site is a three dimensional pocket made by the combination of the VH and VL regions.
Side chains of the amino acids in the pocket play an important role in antigen binding (size, charge of R group).
Covalent bonds require energy and will not contribute to the interactions between the antibody and the antigen.
