Alveolar macrophage subsets: Accessories to lung alloimmune rejection

Canonical pathways of solid organ rejection require donor peptide (typically major histocompatibility complex, MHC) presentation to recipient lymphocytes by one of 3 pathways: the direct pathway, where donor antigen presenting cells (APC) present donor peptide on donor MHC, the indirect pathway where recipient APC present donor peptide on recipient MHC, and the semi-direct pathway where recipient APC present donor MHC and cross-decorated donor MHC. The ability of the host adaptive immune response to recognize and respond to these pathways relies on complex genetic recombination generating a diverse population of T cells capable of responding to novel non-self-peptides.