In most cases, insulin-dependent diabetes results from autoimmune elimination of pancreatic beta cells by T lymphocytes that are generated as a result of complex polygenic interactions between particular MHC haplotypes and non-MHC linked susceptibility modifiers. Immature T cells with potential autoreactivity are normally destroyed in the thymus when they are highly activated after ligation of the T cell receptor (TCR) with "self" peptides bound to MHC molecules on antigen presenting cells (APC) such as macrophages. Here the hypothesis is put forth that non-MHC linked diabetes susceptibility genes contribute to subtle defects in the maturation of macrophages, and in synergy with a diabetogenic MHC haplotype generate APC that are unable to trigger autoreactive T cells to an activation state high enough to induce their destruction.