What is the main cause of graft versus host disease?

Graft versus host disease (GVHD) primarily occurs when mature donor T cells present in the transplanted tissue (the graft) recognize the recipient's body tissues (the host) as foreign. This recognition results in the activation and proliferation of the donor T cells which then attack the host's cells, leading to the characteristic symptoms of GVHD.

This condition is most commonly associated with hematopoietic stem cell transplants, wherein donor immune cells are infused into the recipient. The transplanted T cells can mount an immune response against the recipient's tissues, particularly affecting organs such as the skin, liver, and gastrointestinal tract. In essence, GVHD is a consequence of the immune response mediated by donor T cells, and this is why the activation of donor T cells is regarded as the main cause of the condition.

The other options involve processes that don't directly relate to the development of GVHD. For instance, while infections, autoimmune reactions, and viral infections can cause various complications in a transplant recipient, they do not describe the specific immunological event that leads to the manifestation of GVHD. Thus, recognizing that GVHD is intimately linked to the immune activity of donor T cells is crucial for understanding its underlying mechanism.