Which is a low-weight protein found on the cell surfaces of nucleated cells?

• A. C-reactive protein
• B. B2-Microglobulin
• C. Ceruloplasmin
• D. a2-Macroglobulin

Answer: (B)

A small, low-weight protein on the surface of almost all nucleated cells is the beta-2 microglobulin component of MHC class I. It is about 11–12 kDa and pairs noncovalently with the heavy chain of MHC class I (the HLA-A, -B, -C proteins) to form the molecule that presents peptides to CD8+ T cells. Because MHC I is expressed on nearly all nucleated cells, beta-2 microglobulin is found on their surfaces as part of that complex. The other proteins listed are much larger plasma proteins not bound to cell surfaces as part of MHC complexes—examples include C-reactive protein, a 115 kDa acute-phase protein; ceruloplasmin, about 150 kDa; and alpha-2-macroglobulin, around 700 kDa.