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Lecture: Infection and Immunity
Date: Thursday 8th February
Time: 11am – 12pm
Antibody Structures and Functions
Today we will discuss:
What is the definition of an antigen?
Structures of antibodies and components
Antibody/antigens interactions
Structure and functions of different antibody classes
Production of secreted IgA at body surfaces (protecting areas open to the outside
world, IgA is secreted to protect us from these pathogens).
Monoclonal vs polyclonal antibodies, and their differences.
An antigen is something that can stimulate antibody generation. We produce antibodies in
the bone marrow to make billions of types that can bind to toxins or proteins. On the surface
of a b cell, the cell will become activated and create clones. B cells and T cells are retained in
the memory ready to protect us from pathogens.
Antibody structure
Amino terminus
Fraction crystallisable end
Sometimes we want to adapt an antibody or cut off the Fc portion to produce two Fab
fragments and an Fc portion. In the tumour, the antibody structure may be too large to enter
so by cutting it down this can reach the tumour. To cleave the fragments, we can use papain.
Otherwise, we can use pepsin which breaks down the Fc portion into smaller pieces, leaving
the two arms present (Fab 2).
Affinity & Avidity
If it is just a fab fragment it has a single valance. If we have an IgG with two arms, we can
bind to a pathogen with repeating antigens. The IgM has 10 different antigen binding sites,
with the same fold, improving the chances of holding onto the pathogen.
Monovalent antibody binding site- if it lets go once it will lose the pathogen. As most
antibodies are bivalent, it has a good attachment to pathogens. You need a high affinity to
improve the strength of binding to an antigen.
The interactions holding the antibody to the antigen.
1. Hydrogen bonds, in which a hydrogen atom is shared between two electronegative
atoms.
2. Ionic bonds between oppositely charged residues.
3. Hydrophobic bonds (the strongest bond) – water forces hydrophobic groups
together.
4. Van der Waals interaction with electron clouds. This only comes into effect when
there is a close bond between antibody and antigen.
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