Alloimmunity is a condition in which the body gains immunity, from another individual of the same species, against its own cells. Alloimmunity should not be confused with autoimmunity. Autoimmunity is a condition in which the body's own immune system attacks its own cells.

If either alloimmunity or autoimmunity is directed against red blood cells (RBCs), the direct Coombs test is positive.

Alloimmunity can occur

• in the recipient after transfusions of fluids such as blood or plasma
• in the recipient after allografts (grafts)
• in the foetus after maternal antibodies have passed through the placenta into the foetus, as in haemolytic disease of the newborn and fetomaternal alloimmune thrombocytopenia.

Immunology