For the purpose of prenatal genetic diagnosis, amniotic diagnosis has heretofore been conducted primarily by sampling amniotic fluid via amniocentesis and inspecting chromosomes of the fetal cells in the amniotic fluid. Conventional prenatal genetic diagnostic techniques suffered from the serious problems of a risk of miscarriage in addition to physical and mental stresses on mothers. Under such circumstances, fetal cells (fetal nucleated red blood cells) were found to migrate in the blood circulating in the mother's body. If fetal nucleated red blood cells contained in the maternal blood are selectively collected and the genes of the fetus are analyzed, prenatal diagnosis can be safely carried out without a risk of miscarriage. Such technique enables fetal gene diagnosis at an early stage of pregnancy, which can lead to early treatment. Approximately 5,000,000 cases of prenatal genetic diagnosis are conducted every year on a global scale. If such safe genetic diagnostic technique can be put to practical use, safe techniques can occupy a high share of the global market. However, it is not easy to collect fetal nucleated red blood cells because such cells are said to exist in amounts as small as about 1 cell in 1 ml of the maternal blood. A collection method involving the use of an antibody that recognizes a special structure of the nucleated red blood cell surface (i.e., an antigen-antibody reaction), a method comprising allowing fluorescence-labeled nucleated red blood cells to flow in a liquid, allowing such blood cells to pass through the laser beam focal point, and assaying fluorescence emitted by blood cells to collect cells (i.e., fluorescence activated cell sorting (FACS)), and other techniques have been implemented in research institutes all over the world. However, all such techniques have been insufficient. As a method for collecting nucleated red blood cells with high assuredness, a method comprising analyzing an image observed under an optical microscope and collecting the nucleated red blood cells detected can be employed. According to the FDD-MB® (Fetal DNA diagnosis from maternal blood) project of Takabayashi (Kanazawa Medical University), at present, fetal nucleated red blood cells are separated from the maternal blood via density-gradient centrifugation using Percoll to prepare samples and automatically processed to collect NRBC (Haruo Takabayashi, Idenshi Igaku (Gene & Medicine), Vol. 5, No. 3, 2001, pp. 10-11; Haruo Takabayashi, Idenshi Igaku (Gene & Medicine), Vol. 5, No. 3, 2001, pp. 28-34 2). Detection of the nucleated red blood cells via imaging disadvantageously necessitates a long period of time.
Rare cells have heretofore been separated by density-gradient centrifugation using Ficoll, Percoll, Polymorphprep, or the like (US Patent Publication No. 2003/0134416; US Patent Publication No. 2004/0142463; U.S. Pat. No. 5,714,325; U.S. Pat. No. 6,949,355; U.S. Pat. No. 7,166,443; WO International Publication No. 2008/048931). When such separation reagent is used alone, disadvantageously, nucleated red blood cells cannot be completely separated because their density (specific gravity) is similar to that of white blood cells and some other red blood cells (i.e., 1.07-1.08).