Diversity of the Immune system

Diversity is a hallmark of the immune system. The repertoires of B cells and of CD4 and CD8 T cells each consist of more than 108 different clonotypes each characterized by a unique receptor. Each immune response is characterized by a large panel of different cytokines with –partly overlapping– functions. Each individual is characterized by a unique combination of MHC molecules that play an essential role in the selection of peptides presented to the cellular immune system. MHC loci are the most polymorphic genes known for vertebrates, i.e., for most loci several hundreds of alleles have been identified within each population. However, each individual inherits only a limited number of MHC genes from its parents, and expresses about 10 different MHC molecules. We will here address the evolutionary questions why lymphocytes are so diverse within an individual, and why MHC molecules are diverse at the population level, and not diverse within an individual.