The strength and universality of ambient UVB light and vitamin D3 as autoimmune disease risk factors, the vitamin D hormone’s established role as a transcriptional regulator of gene expression, and the role of CD4+ T lymphocytes as autoimmune disease initiators and suppressors provided the rationale for closely examining calcitriol regulation of CD4+ T-lymphocyte function in this review. Here, CD4 is linked to autoimmune disease.