Class switch recombination: a friend and a foe.

The effector functions of the antibody are determined by the heavy chain constant region (CH). During CSR the primarily expressed mu constant region (Cmu) of the heavy chain is replaced with a downstream isotype Cgamma, Calpha or Cepsilon. The murine immunoglobulin heavy chain (IgH) locus contains eight different CH genes. Class switch recombination (CSR) involves a recombination between two different repetitive switch (S) region sequences, located upstream of each CH gene and the deletion of the intervening DNA. However, this protecting mechanism is also involved in aberrant chromosomal translocations and generation of B cell malignancies. It is also involved in susceptibility to autoimmunity. The current review focuses on the basic mechanism of CSR and the adverse outcomes that it may cause.