A highly negative-charged carbohydrate composed of a linear polymer of alpha 2,8-linked sialic acid residue with potential immunotherapeutic activity. Polysialic acid (PSA) is mainly attached to the neural cell adhesion molecule (NCAM), a membrane-bound glycoprotein overexpressed in certain types of cancers. In embryonic tissue PSA-NCAM is abundantly expressed and PSA plays an important role in formation and remodeling of the neural system through modulation of the adhesive properties of NCAM, thereby reducing cell-cell interactions and promoting cellular mobility. In adult tissue however, the expression of PSA-NCAM is associated with a variety of malignant tumors, signifying its potential role in tumor metastasis. When administered in a vaccine formulation, PSA may stimulate a cytotoxic T cell response against tumors expressing PSA, thereby resulting in a reduction in tumor size.