It is a possible complication of blood transfusions and stands for Transfusion Related Acute Lung Injury. Basically the blood donated has anitibodies in it that attack white blood cells in the recipient. Although the ABO blood grouping system is the most important and widely known there are over 100 other blood antigens that can cause adverse reactions to blood transfusions. When a person has become sensitised to a particular non-self antigen (a marker on a cell that tells other cells what it is) such as if a woman has a child with different antigens and blood mixes during labour, the immune system, much like when fighting an infection, produces memory cells that stay in the blood so that if the same antigen is presented again (i.e. another infection by the same bug, or in this case a transfusion into a recipient with the same type of antigens), a massive and rapid antibody response is produced (hence why TRALI can occur a matter of hours after transfusion). In this case, if the host has got a lung problem, there is relocation of white cells to the lungs as part of a normal response to disease. When the blood is transfused into the patient, the memory cells produce antibodies rapidly in response to these white cells (specifically neutrophils) that have collected in lungs. Because white cells themselves are part of the immune system, they (in response to these antibodies) get activated themselves and start releasing chemicals that have multiple effects to mobilise the immune system (against a bug that isn't even present) and also act on the capillary walls to cause them to constrict (which would normally help to close wounds). This causes an increased pressure in the capillaries which leads to blood components leaking out if the capillaries into the surrounding tissue, causing oedema (tissue swelling), which impairs breathing by increasing the distance oxygen has to diffuse across to reach the blood, in essence suffocating the patient by starving them of oxygen. Therefore, patiens often require ventilatory support to stay alive.