CD14 is expressed mainly by macrophages and (at 10-times lesser extent) by neutrophils. It is also expressed by dendritic cells. It is a component of the innate immune system. CD14 acts as a co-receptor for the detection of bacterial lipopolysaccharide (LPS). CD14 can bind LPS only in the presence of lipopolysaccharide-binding protein (LBP). CD14 exists in two forms, one anchored to the membrane by a glycosylphosphatidylinositol tail (mCD14), the other a soluble form (sCD14). sCD14 either appears after shedding of mCD14 (48 kDa) or is directly secreted
from intracellular vesicles (56 kDa). sCD14 is secreted by the liver and monocytes and is sufficient in low concentrations to confer LPS-responsiveness to cells not expressing CD14. mCD14 and sCD14 are also present on enterocytes. sCD14 is also present in human milk, where it is believed to regulate microbial growth in the infant gut.