visual: heavy chain loci

Creating an annotated bibliography is a great motivator / propeller for finding helpful visuals:

Taken from: Weill, Jean-Claude, and Claude-Agnes Reynaud. 1996. Rearrangement/hypermutation/gene conversion: when, where and why? Immunology Today. 17: 92-97.

This image compares, schematically, Ig heavy chain DNA of sharks, chickens, rabbits and mice. The paper explores the mechanisms used by different species to develop diverse B cell repertoires; 

- sharks have many (cassette-type) clusters of partially fused VDJ sequences, primary B cell repertoire is then generated mostly by hypermutation, intracluster V(D)J rearrangement also takes place

- chickens have upstream pseudogenes that modify the one functional V sequence in both light and heavy chains

- rabbits have ~100 functional and ~100 pseudogene V elements, these all serve as gene conversion donor templates for the V sequence most proximal to the DJ region

- mice have ~9 V regions.

Evolution of B cell diversification: the paper postulates that so long as the requirement for one antibody -> one B cell is satisfied, the means of generating a diverse array of B cell antibodies could vary from species to species.

Interesting fact: a pathogenic microorganism, Borrelia hermsii, employs gene conversion techniques very similar to chicken to generate diverse outer membrane proteins (to avoid detection / neutralization by its host/victim's immune system)