The passerine major histocompatibility complex (MHC) class I and IIB genes are different from those of the avian model species the chicken because passerines have (1) a larger number of MHC genes, (2) MHC genes with longer introns, and (3) MHC genes that are pseudo-genes. Most passerine MHC genes are transcribed (coding), extremely variable and subject to balancing selection. The high genetic diversity of the MHC genes of passerines is most likely maintained by selection from a large number of different pathogens. Association between MHC alleles and resistance to avian malaria infections have been reported in House Sparrows and Great Reed Warblers. Passerines are infected by a large number of different avian malaria infections. Therefore passerines and avian malaria is a study system that is well-suited to investigations of balancing selection and associations between MHC genes and disease resistance.