I'm just wondering why MHCI specifically would be involved in allograft rejections... such as a kidney.

 

My understanding is that since MHCI is found on all nucleated cells, it would be quite prevalent throughout the human body system, therefore easily detecting any foreign molecule.

 

Then how would MHCII be involved in transplant rejections as well?

Tissues are regarded as 'non-self' if it is present by nonself MHC. Class I MHC activates Tc lineage and class II MHC activates Th lineage, the former requires support from the latter. When class I MHC is recognised by a Tc cell and it differientiates to CTL with the help from Th cell after recognising foreign class II MHC. CTL attacks and rejects the transplant. The process is also accompanied by B cell lineage attack involving Ig as well as complement system.

Hi Apoptosis,

 

Organ rejection can be mediated by both antibodies and by T cell responses. B cells recognise both non-self MHC Class I and II molecules. If a patient has had prior sensitisation, e.g. previous transplants or blood transfusions, the rejection can occur very quickly and is usually mediated by antibodies. T cell mediated rejection occurs as a result of CD8 T cells recognising the non-self MHC Class I molecule too well. Within the thymus new T cells that bind self MHC Class I molecules too well are deleted. However, as non-self MHC Class I molecules are not present within the thymus this selection can not happen. The T cell mediate rejection tends to be much slower as it take time to generate a response.

 

Hope that helps!