Cells of biomedical interest are often present in very small numbers (i.e. they are rare), despite their functional significance. The analysis and isolation of previously inaccessible rare cells, such as peripheral hematopoietic stem cells, fetal cells in maternal blood, residual tumor cells or antigen-specific lymphocytes, has now become feasible through the development of new methods. Today, these techniques allow the detection, isolation and analysis of cells less frequent than one in a million. Not many problems in immunology would require higher resolution.