**Author details**

DNA polymerase Polη has no exonuclease activity and is error prone in B cells. The polymerase preferentially misincorporates thymidine (T), regardless of the template sequence, which leads to a preference of adenosine (A)-thymidine (T) mutations at the original targeted

Alternatively, in the base excision repair pathway, the uracil DNA glycosylase (UNG) cleaves the uracil nucleobase from the uridine and leaves an abasic site in the DNA strand. During the following DNA replication, a random DNA base will be inserted in the opposite DNA strand of the abasic nucleotide. This is mediated by an error-prone DNA polymerase used in transle-

As mentioned before, AID can also initiate class-switch recombination, by acting of apurinic/ apyrimidinic endonuclease 1 (APE1) upon UNG-mediated introduction of an abasic nucleotide in the switch region. APE1 cleaves the DNA strand at the abasic site and produces a single-strand nick. In the switch regions, upstream of the constant region genes, the DNA nick is further cleaved which produces a double-strand break (DSB). This leads to a joint of another constant region gene to the V region, produced by the double-strand break repair machinery.

In naive B cells, which had already rearranged their V region by somatic DNA recombination, two antibody isotypes are co-expressed at the same time. The V region and the μ chain (IgM) together with the δ chain (IgD) were transcribed on the same RNA transcript. By alternative splicing, either the μ chain or the δ chain is chosen, which produces two different messenger RNAs (**Figure 1**). Upon antigen contact and B-cell activation, B cells switch their antibody isotypes from IgM/IgD to IgG, IgA, or IgE. This is achieved by a process called class-switch recombination (CSR) or isotype switching. The antibody isotype is changed by an exchange of the constant region of the heavy chain locus. Only the constant region is replaced by CSR, which means the V region stays the same, but class switch confers the antibody the ability to interact with different effector molecules by their fragment crystallizable (Fc) region (**Figure 5**).

cytosine and the adjacent nucleotides by the mismatch repair pathway.

sion DNA synthesis for damaged DNA caused by UV radiation.

**3.2. Class-switch recombination**

12 Antibody Engineering

**Figure 5.** Mechanism of switch recombination from IgM to IgG2b.

Oliver Backhaus

Address all correspondence to: olli.backhaus@googlemail.com

Institute of Pathology, University Clinic of RWTH Aachen, Aachen, Germany
