Antibodies, compositions, systems, and methods for detecting C4.4a, for example immunohistochemistry methods for detecting C4.4a using a C4.4a antibody. The antibody may be obtained by immunizing a host with a C4.4a protein such as a peptide downstream of the signal peptide. The antibodies may be adapted to detect the uPAR-like domain 1 and uPAR-like domain 2. Also featured are methods for diagnosing C4.4a-associated tumors using C4.4a antibodies disclosed herein.