The heavy chain switch in antibody production is critical to effective protection against pathogens. The heavy chain switch is mediated by a DNA recombination event between switch regions. Switch regions are 2-8 kb long stretches of simple sequences repeated in tandem that lie upstream of each heavy chain gene [except delta). Recombination between two switch regions must be efficient so that large quantities of a given type of antibody can be produced. The efficiency of switch recombination is thought to be regulated by four enhancer elements that lie 3'of the heavy chain constant region locus. In Aims 1 and 2, we will test how deletion of various combinations of the four enhancer elements alter the efficiency of switch recombination. Each of the seven types of secreted antibodies are designed for the optimal neutralizing, clearing, or milling of a subset of pathogens. Hence, the heavy chain switch must be regulated in a heavy chain-specific manner in order to produce a heavy chain for the best protection against a given pathogen. The heavy chain switch is preceded by opening of the chromatin for a specific heavy chain gene, which is best measured by the expression of germline transcripts before the switch recombination event. Germline transcription and DNA recombination between switch regions are regulated by cytokines. In Aim 3, we will determine if cytokine regulation of germline transcription and switch recombination to a specific gene is regulated by DNA sequences in the promoter region for germline transcripts or by the switch region sequences. The experiments in all three Aims will be carried out by testing various mutant forms of a transgene of the entire heavy chain constant region locus. The ability to mutate this recombinant DNA molecule in E. coli, and then test its ability to mediate germline transcription and switch recombination of all seven transgenic heavy chain genes in normal B lymphocytes, offers several advantages compared to other approaches. Regulated heavy chain switching is at the center of several practical immunological problems. Vaccines should be designed to elicit the largest amounts of the most favorable types of antibodies. Almost alll allergies are caused by the expression of one type of antibody. The understanding to be gained from the proposed experiments may improve approaches toward vaccine development and toward treatment of allergy.