B-lymphocytes and antibody Flashcards
Describe the structure of an antibody.
Consists of 4 chains: 2 heavy, 2 light
FAB region consists of heavy and light chains. It is the antigen binding site.
Fc region is the alternate binding site for other components of the immune system e.g. B cell.
Heavy chains held together by disulphide bridges
What is the epitope of an antigen?
It is the specific site at which the antigen binds an antibody. The basis of this binding is physio-chemical affinity.
Consider the antibody isotopes. Describe immunoglobulin-M
First antibody produced in immune response
Relatively low affinity for antigen
Immature
Forms pentamer to increase affinity by increasing SA.
Pentamer involved 5 antibodies around a J-protein
Consider the antibody isotopes. Describe immunoglobulin-E
No known function
Monomer
Consider the antibody isotopes. Describe immunoglobulin-A
Found in breast milk
Can cross mucous membranes so found in gut, tears, saliva and other secretions.
For dimers with J proteins with s chain that protects it against secretory elements.
Consider the antibody isotopes. Describe immunoglobulin-G
Main mature antibody
Monomer
Consider the antibody isotopes. Describe immunoglobulin-D
No known function
Immature
How do antibodies help us?
- Binding directly (toxin neutralisation and receptor blocking)
- Binding then interacting with another element of immune system e.g. phagocytes, complement, mast cell activation, antibody-dependent cellular cytotoxicity
- Act as B cell receptor when bound to B cell
How do phagocytic cells increase efficiency of phagocytosis?
They present receptors for the Fc portion of the antibody to decrease repulsion between the two negatively charged membranes.
What is opsonisation?
Process of coating bacteria to enhance phagocytosis.
Describe mast cell activation
Have surface Fc receptors
Become coated with IgE from circulation
When antigen binds to IgE and cross links them, the mast cell degrades
Degradation of mast cell releases histamine
Important in parasitic infections
Inappropriately triggered in allergies
Describe antibody-dependent cellular cytoxicity.
A NK cell expressed a receptor that is complementary to the Fc portion of an antibody of an antibody-coated bacteria.
Leads to destruction by non-phagocytic means
How is the variable region of an antibody made?
By somatic recombination.
This involves selection of different gene segments thought each DNA region and combing them.
Suggest 4 advantages for somatic recombination
Huge diversity
Able to make a large number of receptors from a small are of DNA
Unique repertoire- résilient in different environment
Gene segments are inherited so there is some evolutionary benefit
Suggest 2 disadvantages for somatic recombination
Randomness means some combinations don’t work because they 1. biochemically don’t fit 2. bind to our own proteins (self antigens)
B cells with dysfunction receptors mainly destroyed- this is energy intensive
Affinity maturation occurs once an antigen has been met. This occurs by class switch and somatic hypermutation. Describe each
Class switch- IgM in primary response switches to IgG. Variable region of the antibody remains the same.
Somatic hypermutation- random mutations introduced into variable region so daughter cells produce ‘better fit’ antibodies. THIS IS INEFFICIENT
What are antibody isotopes?
Antibodies with different heavy chain constant regions and so have different properties.
