IGHA2 and vitamin D deficiency: Furthermore, more differential proteins were identified between groups A and C, and vitamin D deficiency upregulated the expression of 8 proteins and downregulated the expression of 17 proteins (Fig. 2E, A vs. C), such as CFB, immunoglobulin heavy constant alpha 2, complement component C9, peptidase inhibitor 16 (PI16), and ICOSL.