Lecture 29 - B Cells. Flashcards
What is the basic structure of an immunoglobulin (Ig)?
- Y-shaped glycoprotein made up of:
-> 2 identical disulfide-linked heavy (H) chains.
-> 2 identical light (L) chains.
What protein family do antibodies and B-cell receptors belong to?
Immunoglobulin (Ig) family (Ig-like domains)
How many chains are in an immunoglobulin?
- 4 chains
2 heavy (H) chains
2 light (L) chains
What are the two main regions in both the heavy and light chains of an Ig?
Variable region and constant region
What are the specific names for the variable and constant regions in the light chain?
VL – Variable region in light chain
CL – Constant region in light chain
What are the specific names for the variable and constant regions in the heavy chain?
VH – Variable region in heavy chain
CH – Constant region in heavy chain
What forms the antigen-binding site in an Ig?
VL + VH (2 identical antigen-binding sites per antibody)
What is the function of the variable regions (VL and VH)?
Antigen binding → leads to neutralization and other immune functions
What is the function of the constant region (CH)?
- Complement activation (C1q → classical pathway)
- Fc region can bind to Fc receptors on phagocytes and other cells (e.g., mast cells, eosinophils)
Fab fragments
- Two Fab fragments per antibody
- Each has antigen-binding domain AND part of the
constant H and L chains - Fab = fragment antigen binding
Fc fragment
- One Fc fragment
- Fc = fragment crystallizable
- Constant region of the heavy chain
- Receptors that bind antibodies recognize the Fc region
What is the three-dimensional structure of an immunoglobulin (Ig)?
- Two heavy chains (variable and constant regions)
- Two light chains (variable and constant regions)
- Held together by intra-/interchain disulfide bonds
- Constant and variable regions are folded into complex 3D structures with β strands
What is the complementarity determining region (CDR) in an antibody?
- Part of the variable region → forms the antigen-binding site
- Direct contact with antigen (Ag)
What are hypervariable loops in the CDR?
- 3 loops per variable domain
- Not part of the β strands
Where is the greatest variability in the antibody sequence?
In the CDRs
Where are the CDRs located on the antibody structure?
At the extremities of the antibody
How does antigen binding occur in immunoglobulins?
- Through non-covalent bonding between the Ig and the antigen (Ag) epitope:
1. Hydrogen bonds
2. Van der waals
3. Hydrophobic
4. Ionic
What principle describes the specificity of antigen binding?
Lock and key specificity
Which part of the antibody is responsible for antigen binding?
The extremities of the antibody
What factors influence the recognition of an antigen by an antibody?
- Size variability of the antigen being recognized
- Variation in the antibody itself, with CDR varying in length
Where can epitopes be located on an antigen?
Epitopes can be located anywhere on the antigen
What are the five major classes of antibody?
IgM
IgD
IgG
IgE
IgA
What are some key differences you observed between the different antibody
classes?
- Different number of Ig-like domains – differences in the length of the constant
region of the heavy chain. - Differentiated by amino acid sequence
of heavy chain (constant region)
-> Heavy chain is what differentiates
the different antibodies
-> Fc fragment of each Ig is different - Each class performs different functions during immune responses.
IgM
- Pentameric
- 5 antibodies linked together via disulphide bonds
- Heavy chain: one variable region and 4 constant
regions - Mature naïve B cells express transmembrane IgM
- IgM is part of first wave of secreted antibodies
- Most effective initiator of complement cascade
