HLA system in solid organ transplantation part 10

Pathways of allorecognition of the alloantigens

As mentioned previously, antigen-presenting cells (APC) play an important role in activation of alloreactive T cells. APC provide the three necessary signals for T cell activation:

• MHC (peptide complex)
• Costimulatory molecules
• Inflammatory cytokines

They also find different ways to allostimulate T cells depending on the origin of the APC and the source of the MHC molecules, the so-called pathways of allorecognition:

• Direct
• Indirect
• Semi-direct