C3 and diabetic neuropathy: A recent review reported that high complement protein C4A copy numbers and low C4B copy numbers are protective against residual β-cell function (Ajjan and Schroeder, 2019), and a clinical trial involving 95,202 participants has indicated that high baseline concentrations of complement C3 were associated with increased risk of diabetic neuropathy highlighted that complement component C3 has a role in the pathology of diabetic neuropathy (Rasmussen et al., 2018b).