Complement C3d is a 187148 Da extracellular immune protein expressed in aorta, brain, cerebrospinal fluid, glomerulus, heart, kidney, leukocyte, liver, monocyte, plasma, salival, semen, serum, skin fibroblast, spleen, t cell, temporal cortex, and urine; Antibodies to Complement C3d can be used to study immune response. Complement C3d, also known as C3d, is a designation for C3.