By introducing human immunoglobulin loci (heavy and light chains) into the mouse germlines with inactivated/disrupted endogenous antibody machinery, the transgenic mouse/rat strains can directly generate high affinity, fully human antibodies, which avoid the subsequent costly and tedious in vitro humanization and affinity maturation process. Upon immunization, the Ig transgenes will undergo V(D)J rearrangement, random nucleotide (N region) addition, class switching, and somatic hypermutation to yield impeccable antibodies. This technology takes advantage of the naturally intact immune system with robust response, high-efficient selection and in vivo maturation while maintaining fully humanness which can eliminate immunogenicity concerns.

http://www.creative-biolabs.com/Human-Antibody-Production-from-Transgenic-Mice.html