Figure 4
From: Mathematical framework for human SLE Nephritis: disease dynamics and urine biomarkers
Hill functional of order 2. We represent the immune complexes (I) formation due to accumulation of self-antigens from debris D, by a Hill functional of order 2, 
. When there are only a few antigens around, not many immune complexes are produced; as antigens accumulate, more immune complexes are being created, and saturation, s
id
, is reached for sufficiently many self-antigens.