Antibody Structure Problem Set

Problem 6. Antibody specificity

Tutorial to help answer the question

Replacing the antigen's glutamine 121 with histidine greatly decreases the affinity of the antibody for lysozyme. The best explanation for the difference in affinities is that histidine can't fit into the depression occupied by glutamine. An important implication of this observation:

A.	minor changes in antigen structure can dramatically change immune recognition
B.	pathogens that mutate frequently are more difficult for the immune system to eliminate
C.	A and B are true

Tutorial

An antibody's 3-D structure, determined by its amino acid sequence, gives the antibody specificity against one antigen. If that antigen structure changes, the antibody will most likely bind with less affinity to the antigen. The glutamine 121 in hen lysozyme is an important part of the antigen epitope because the formation of hydrogen bonds to this exposed amino acids allows further formation of hydrogen bonds and van der Waals contacts with the rest of the epitope. Even though the epitope is spread out over the surface of the antigen, a few residues like glutamine 121 are critical for antibody binding.

The fact that a single amino acid change can alter antibody specificity means that viruses with unstable genomes are more difficult to eliminate by an immune response. HIV, a retrovirus, is a good example of a virus with an unstable genome which eludes the body's antibody-based, or specific immune response.