The main cause of a negative response to grafting is immune rejection connected with a reaction to the donor's antigens. The process of rejecting transplanted organs is a whole-body process. The reaction of the organism begins when the donor's antigens reach the recipient's lymphatic organs via blood or lymph. The donor's genetic material (DNA) is detected in the recipient's blood and lymphoid tissues even a few months after transplant rejection. Microchimerism occurs when in an individual patient the cells and genetic material from both the donor and recipient are present and the cell count of one of these is overrepresented. Such situations can occur after blood transfusions, grafts or pregnancies. It is suggested that this phenomenon could have an influence on tolerance, prolonged graft survival or the rejection process. The results of many experiments are thus far equivocal; therefore further research is needed elucidate the process and mechanisms of graft rejection and its molecular aspects.