Engraftment means the time for the red cells, white cells, and platelets to appear in the blood. The most important cell to return is the neutrophil. The neutrophil is one type of white cell and is important in fighting bacterial infections. The neutrophils are usually the first cells to return, especially if a growth factor, like G-CSF, is given. The platelets are next to return.
Platelets are fragments of cells that live in the bone marrow.
They are important in stopping you bleeding when you are cut.