  Although Vitamin D is believed to be involved in a large number of immune
responses our understanding of these processes at cellular level remained at
infancy. We develop and solve a coarse grained kinetic network model to
quantify the effects of variation of vitamin D on human immunity. The system of
equations accounts for known inter-relation between active and inactive vitamin
D, antigen presenting cells, effector T cells, regulatory T cells and pathogen.
Both time dependent and steady state solutions are obtained. The time dependent
solution of the system of equations reveals that the immune response is rather
strongly regulated in presence of vitamin D. We found quantitatively that lower
than optimum levels of concentration of active vitamin D correspond to weak
regulation where, once a pathogen/antigen enters the body, the nature of the
immune response would be less regulatory and hence more aggressive, or
inflammatory. The steady state solution of our model shows that vitamin D
enhances the tolerance level of immune system, thereby increasing resistance to
autoimmune diseases. Our model and accompanied numerical analyses reveal
another important aspect of immunity: While extremely low levels of vitamin D
could lead to increased risk of autoimmune responses, an overdose (toxic) level
would give rise to too large a tolerant response, leading to the increased risk
of tumors and cancerous cell growth.
