Lecture 4- recognition of antigens Flashcards
What is an essential feature of the adaptive immune response?
The generation of a large repertoire of molecules that will specifically recognise a wide variety of antigens. This is achieved by cells of the lymphoid lineage only (B cells and T cells)
What do B and T cells produce?
Structurally highly related antigen recognizing molecules (immunoglobins). This entails somatic recombination.
What can bind free antigens?
B cells. T cells cant they require antigen presenting cells (myeloid and lymphoid cells). Express major histocompatibility complex molecules on surface.
What is the relationship between antibody and B cell receptor?
Almost identical except for a difference in the C-terminus of the heavy chain constant region. BCRs have a hydrophobic membrane-anchoring sequence that keeps them attached to the B cell membrane. Antibodies have a hydrophilic sequence that allows them to be secreted into the blood stream.
What is the epitope?
Antigenic determinant. The specific part of the antigen that is recognised by the BCR, antibody or T cell receptor.
What is the paratope?
The specific part of the antibody that binds to the epitope of the antigen.
How does papain cleave antibody fragments?
Into two Fabs and an Fc.
How does pepsin cleave antibody fragments?
F(ab’)2 and pFc’
What is the structure of the V (variable) and C (constant) domains on the light chains?
Consist of two beta sheets which are formed from several strands of a polypeptide chain in extended conformation. Two antiparallel beta sheets are linked by disulphide bridge and form a roughly barrel shaped structure (beta barrel) termed Ig domain. This distinctively folded structure is known as the Ig fold.
What is the Ig superfamily?
Not only antigen binding (antibodies B and T cell receptors but many other functionalities e.g., DNA binding).
How does the structure of heavy chains compare to the structure of light chains?
Conservation. 3 domains of hypervariable regions where the antigen binding site is. 3 domains of framework regions that form beta sheets for maintaining structural framework. then a complementary determining region as antibodies only bind to antigens whose surface is complementary to it.
What is combinatorial diversity?
Different combinations of heavy and light chain variable regions produce variability. A second form of variability is created from rearrangement of small segments of Ig-encoding DNA somatic recombination (L5).
How many domains are in the heavy chain?
CH1, CH2, CH3 and VH
What is at the C terminal domain?
Constant region not involved in antigen binding.
What is at the N terminus?
Variable antigen binding site.
Where is each class of Ig produced?
IgG first discovered, most abundant in serum, 4 domains in heavy chain. Sub classes of 1-4.
IgG found in macroglobulin fraction of serum. IgA found in mucosal linings/secretions. IgD product of a myeloma (antibody producing tumour). IgE is a pathogenic antibody in the serum of allergy sufferers.
What are high order polymers?
IgM and IGA form them. Polymerisation requires a tailpiece of 18 amino acids in the C region that contain a cysteine residue. J chain links to the tailpiece. IgM pentamer and IgA dimer have one J chain. Polymerisation is important in binding to repetitive epitopes. IgA polymerisation is required for transport through the epithelia.
