Compositions and methods are described for highly sensitive quantification of the relative representation of DNA from adaptive immune cells (e.g., T and/or B lymphocytes) in DNA extracted from complex mixtures of cells that include cells which are not adaptive immune cells. Included are methods for determining the relative presence in a tumor of tumor infiltrating lymphocytes (TIL), the relative presence of lymphocytes infiltrating a somatic tissue that is the target of an autoimmune disease, and the relative presence of lymphocytes infiltrating a transplanted organ.