Songbirds include more than half of all bird species on earth and their enormous multiplication in species numbers indeed shows that they have the ability to adapt to different environments. Long distance migratory songbirds that breed at our latitudes manage to overcome all pathogens that they are exposed to in tropical Africa at their winter quarters but also the completely different pathogens faunas at their stop-over and breeding sites. Such songbirds may have particularly well developed immune systems and it would be rewarding to know what proportion of their immunity genes (so called Major Histocompatibility Complex genes (MHC)) that is of significant importance in adaptive immunity.  The central aim of this proposal is to characterize the MHC genomic region in two songbird species, species that are distant in the songbird phylogeny, the great reed warbler Acrocephalus arudinaceus and house sparrow Passer domesticus, and also to measure gene expression per gene and study antigen binding properties of MHC proteins encoded by genes with different degrees of expression; the key questions are: a) How many MHC gene copies are there in songbird genomes? b) Are all MHC genes found in a core MHC region or are they distributed throughout the genome? c) How does the degree of expression vary between different MHC gene copies and across tissues? d) Are there particular MHC genes at certain loci that are expressed to a higher degree than others? e) To what extent does the antigen binding preference of different MHC proteins differ between different MHC gene copies?