Respiratory Syncytial Virus (RSV) is the major cause of acute lower tract infection in early childhood. Annual epidemics occur which are well documented in developed countries during winter months, placing considerable pressure on the provision of health care. Little is known about the epidemiology of RSV infection in the Middle East and other desert climate regions of the world. The aim of this project was to study the specificity of the immune response to RSV B in children, and to relate this to the infecting RSV strain, with particular emphasis on antibody response to RSV attachment (G) protein. RSV is an important cause of hospital admission in children (54%) during winter months in Qatar. 63% of these infections are due to RSV A. A case study involving analysis of RSV strains from hospitalized children was carried out in Qatar over 2 winter seasons (1999-2000 and 2000-2(01). RSV incidence was found to correlate with high relative humidity and low temperature. A comparison of enzyme immuno assay (EIA) and polymerase chain reaction (peR) for the detection of RSV in clinical samples demonstrated that peR was more sensitive than EIA. All positive RSV samples obtained during the study period were classified as belonging to RSV A and B subtypes using Multiplex peR. In this project, primer sets were designed and optimized to amplify the whole of the G gene of RSV A and B strains. Derived sequence analysis allowed deduction of the molecular epidemiology of RSV G gene for RSV A and B strains in Qatar and elsewhere. Sequence data of the G gene from Qatar RSV A&B isolates confirmed the variability of this protein and showed that variability occurs among group B RSV viruses isolated in Qatar (0.8%), although to a lesser extent than among the group A viruses (5%) from same location. However, the group B viruses isolated in Qatar were highly variable in G gene sequences compared to the prototype strain RSVB N2 (13%) and to strain world-wide (10%) at the nucleotide level. In order to produce an epitope map of the RSVB G protein, synthetic peptides representing linear B-cell epitopes of a representative Qatar RSV B isolate (B/Q/28/00) were used. The reactivity of human sera with the synthetic peptides was studied using sera from young children from whom RSV had been isolated. The sera from these children had variable peptide binding responses against different regions of the G protein of RSV B and the responses were focused on the conserved region. The results indicated that peptide 14 (a. a 150-165) of the G protein, in the conserved region of the protein, is a major antigenic site. This peptide sequence was recognized by a majority of the tested sera (93% sera). To examine the relationship between neutralization antibody titre and reactivity to the synthetic peptides in children's sera, a modified micro-neutralization method was used. There was no significant correlation between the peptide binding activity in the sera and the neutralization titre of these sera. Sera from children infected with RSV A bound to peptides from the G protein of RSV B strain. However, children infected with RSV B had greater neutralizing antibody titre to RSV B strain than to the two RSV A strains. There was no difference in the neutralization antibody titre in sera to RSV A when assessed with prototype strains isolated many years ago (RSV A2), and with the recently-isolated strain AlQ/IO/OO.