B cells and Humoral immunity - Hudig Flashcards
What is the difference between humoral immunity and cellular immunity?
In humoral immunity, antibodies are the effectors of protection.
What is affinity as applied to antibodies?
A measurement of the binding strength of a single Fab antigen-binding site for its single antigenic epitope. It is expressed as Ka. Ka=1/Kd. Kd is the molar concentration of an antigen that is required for 50% of antibody sites to be filled.
What is avidity as applied to antibodies?
A measurement of the total combined binding strength of a complete immunoglobulin molecule for a complex antigen that contains repeating epitopes. For example IgM is a multimer and it has 10 binding sites for antigen so it has high avidity.
When does antibody affinity matter?
In events when you have competition such as when you give a monoclonal antibody for therapy or when you want rapid clearance of a toxin - such as tetanus. This toxin is so potent that you want even a small amount of antibody to be able to clear it.
Describe the nomenclature for isotopes.
- English letters are give for classes - MADGE. Each class has its own type of heavy chain. For example IgE will have a different heavy chain than IgM.
- These english letters correspond to greek letters for heavy chains - IgA - alpha
- IgM - mu
- IgD - delta
- IgG - gamma
- IgE - epsilon - numbers are give for different isotopes of the various classes. IgA and IgG have isotypes in humans.
Describe an Ig monomer.
Each monomer is made up of 2 identical heavy chains that have 4 domains (usually 3 are constant and 1 is variable) and 2 identical light chains that have 2 domains (one is constant and one is variable). The Heavy chains determine what class the antibody is in. The variable regions are involved with antigen binding.
What are the biological features of IgM?
IgM is a pentamer, 5 Ig molecules are joined via disulfide bonds with a J chain that is like a clasp to hold the multimer together. I t has a serum concentration of 1.2 mg/mL and a half life in serum of about 5 days. It is the first antibody made after antigen is introduced and has a high avidity/low affinity for antigens. Molecular weight is 750 kD. It has a membrane form on the surface of virgin B cells.
What is IgM good for?
It fixes complement well and is very important in clearing bacteria and yeast from the blood.
How many binding sites does an IgM pentamer have?
10
Where is IgM found?
In the serum and in secretions such as tears and gut secretions.
Describe IgG.
- IgG’s are monomers
- there are 4 different subclasses/isotypes #’s 1-4
- The AA sequence is different in the constant regions and hinge regions for the various subclasses
- they are long-lived
- they are the second antibody made in immune responses
- they have high affinity/low avidity
- they have a serum concentration of about 7-12 mg/mL
- half - life of IgG1, IgG2 and IgG4 is 21-24 days and 7-8 days for IgG3
- IgG1 and IgG3 fix complement - need a minimum of 2 to start cascade
- only IgG crosses the placenta
- IgG1 and IgG3 support ADCC
- molecular weight is 150 kD
What is ADCC?
A mechanism of cell-mediated immune defense whereby an effector cell of the immune system actively lyses a target cell, whose membrane-surface antigens have been bound by specific antibodies. It is one of the mechanisms through which antibodies, as part of the humoral immune response, can act to limit and contain infection.
Describe IgA.
- Forms a dimer or trimer connected via a J chain.
- Has 2 subclasses # 1-2
- molecular weight of 150 kD per monomer
- serum concentration of 2 mg/mL
- half life of about 6 days
- blocks pathogens at mucosal sites
How many binding sites does IgG have?
2
How many binding sites does IgA have?
4 (dimer) or 6 (trimer)
