L3: Antibodies and Antigens Flashcards
How are antibodies bifunctional molecules?
One end interacts w/ foreign antigen (can interact w/ at least 2 epitopes)
Other end interacts w/ components of host defenses (Fc receptors and complement proteins)
Where are antibodies found?
Serum and tissue fluids
Bound (noncovalently) to Fc receptors on the surface of many cell types
Integral membrane protein on the surface of unstimulated B cells (the only place that antibodies exist as integral membrane proteins)
What happens when an antigen binds to the B cell?
It differentiates and matures and becomes a plasma cell, meaning it is an antibody factory
What are antibody effector functions?
Opsonization: coating on bacteria which makes it easier to digest by phagocytes
Activation of complement
Antibody-dependent cell-mediated toxicity (ADCC)
Ab transport through epithelium or placenta: this is how passive immune protection is passed from mother to fetus
Activation of mast cells, eosinophils and basophils by IgE: these are involved in hypersensitivity reactions, asthma, and allergies
What is the basic structure of all antibodies?
2 identical light chains and 2 identical heavy chains
Light chains have one variable domain and one constant domain
Heavy chains have one variable domain and either 3 or 4 constant domains (depending on class of antibody)
What are the types of light and heavy chains of antibodies?
Light: lambda or kappa (about a 50/50 mix)
Heavy: alpha, delta, gamma, epsilon, or mu (each type is specific for an antibody class such as IgG, IgM, etc.)
How are the light chains held to the heavy chains in antibodies?
Disulfide bonds
How are the 2 heavy chains of antibodies held together?
Disulfide bonds
How are the individual immunoglobulin domains (within the chains) held together in antibodies?
Disulfide bonds
Where are there carbohydrates in antibodies? What do they do?
In the second constant domain
They help w/ solubility and protection from proteolytic digestion
What are hinge regions of antibodies susceptible to?
Being cut by bacterial protealases
What is the structure of immunoglobulin domains?
Antiparallel beta sheet structure
Why is beta sheet structure of immunoglobulin domains favored?
Although the beta sheet is limited (bc have to use small aa’s in beta sheet), the loops can have almost any aa’s bc the R groups are sticking out into the environment
Where is there the most variability in antibody structure?
In the complimentary determining region of variable domain of the light chain
This is where antigen interacts
What do the hinge regions of immunoglobulin do?
They impart flexibility and allow both arms of the antibody to interact w/ an epitope
Important for allowing antibody to bind and stay there for a long time
Epitope
The region of the antigen where the antibody binds
Paratope
Surface of the antibody where the epitope binds
What do secreted immunoglobulins bind to?
Bind to Fc receptors on the surfaces of many cell types; this is non-covalent binding
Antigen receptors
Immunoglobulins that are integral membrane proteins present on unstimulated B cells
What does binding of antigen to immunoglobulins on unstimulated B cell cause?
Activates the B cell and causes them to proliferate AND to produce the secreted form of the antibody that they expressed on their cell membranes?
How are membrane immunoglobulins locked into the membrane?
They have a hydrophobic transmembrane helix w/ polar groups at each end
What is the difference b/w membrane and secreted immunoglobulins?
These forms differ only at the C terminus
What causes the difference b/w the membrane and secreted immunoglobulins?
Difference is caused by changes in mRNA splicing (resulting in the use of a different polyadenylation site)
When immunoglobulin gets secreted, the entire transmembrane helix region gets replaced w/ a very small glycine-leucine-COOH
mRNA is transcribed and in RNA processing, there are 2 places where PolyA tail can be put on; if B cell is activated, polyadenylation factor gets upregulated and it will bind and direct the mRNA to use an earlier polyadenylation signal, which cuts off the transmembrane helix
What are the 5 immunoglobulin classes in humans?
IgG - gamma heavy chain IgM - mu heavy chain IgA - alpha heavy chain IgE - epsilon heavy chain IgD - delta heavy chain
How many immunoglobulin subclasses are there?
IgG, most common, has 4 subclasses
IgA has 2 subclasses
All others have 1
Characteristics of IgG
Major human serum immunoglobulin (60 - 70% of total circulating Ig)
Principal antibody mediator of the secondary immune response
Only antibody class that can cross the placenta and provide passive protection to the fetus
Primary antibody class used in research, diagnostics, and therapeutics
How do the 4 IgG subclasses differ?
Are 90 - 95% homologous in DNA sequence
Only real difference b/w them is the length of the hinge region and the numer and position of the interchain S-S bonds
Why is IgG3 particularly flexible and good at fixing complement? What is the disadvantage of this?
Has a long hinge region, so there is lower chance of steric hindrance when binding to antigen on one end and then binding to Fc or complement receptors on other end
Disadvantage is that teh long hinge region is also good for proteases and will get clipped
What are the relative serum concentrations of the IgG subclasses?
IgG1 > IgG2 > IgG3 > IgG4
Normal adult serum concentration of IgG
12.5 +/- 3 mg/ml
What are abnormal IgG serum concentrations? What can they indicate?
Below 0.6 mg/ml - immunodeficiency, monoclonal gammopathy of IgA or IgM
Above 17 mg/ml - Infections, liver diseases, autoimmune diseases, cystic fibrosis, sarcoidosis, polycythemia cera, 3rd trimester of pregnancy, myeloma, monoclonal gammopathy
Above 50 mg/ml - myelom, monoclonal gammopathy; is cancer or pre-cancer at these high ranges
Characteristics of IgM
10% of total antibody pool, almost all in circulation
Primary antibody mediator of the primary immune response
Agglutinates antigen - increases antigen visibility
Proficient at complement fixation