case_images: PMC10925646_01
case_images: The patient's clinical course. YOA, years of age; URTI, upper respiratory tract infection; LN, lupus nephritis; MTX, methotrexate; MPA, mycophenolic acid.
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case_images: PMC10925646_01_fimmu-14-1257525-g001.jpg
case_images: f1
case_images: His symptoms started at the age of 13 years and are illustrated in  Figure 1 .
case_images: Microscopy study of kidney biopsy. (A) Periodic acid-Schiff staining (magnification x40) disclosed global mesangial hypercellularity and matrix expansion. (B) Electron microscopy (EM) showed subendothelial deposits. (C) Immunohistochemistry revealed deposition of C1q, (D) C5-9, and, to a lesser degree, (E) IgM in the mesangium and along the glomerular basement membrane.
case_images: fimmu-14-1257525-g002.jpg
case_images: PMC10925646_01_fimmu-14-1257525-g002.jpg
case_images: f2
case_images: A kidney biopsy was performed, allowing the visualization of nine glomeruli ( Figure 2 ).
case_images: Functional complement assays and C1s reconstitution. Two healthy donors, two C1q-negative patients, and a C1s-negative patient were investigated for the activation of the classical complement pathway by an ELISA-based assay determining C4d deposition (dotted bars). Complement activation was investigated again after substitution of C1s proenzyme (dark gray bars). NHS, normal human serum.
case_images: fimmu-14-1257525-g003.jpg
case_images: PMC10925646_01_fimmu-14-1257525-g003.jpg
case_images: f3
case_images: The suspected diagnosis of C1s deficiency was confirmed by C1 reconstitution experiments, in which externally added C1s could restore the complement activation in the patient's serum ( Figure 3 ). The results are shown in  Figure 3 .