The primary disadvantage of cord blood as a source of cells for transplantation , the number of cells is small , usually less than 1 billion times, allowing operation only in children under 30 kg , while the majority of recipients has a larger weight. Hence the idea of simultaneous use of several units of cord blood , which together are designed to provide a sufficient number of cells for transplantation to an adult recipient. The bottom line is that the immune cells in cord blood , as young cells should not be able to cross- reactions against each other.