Bjorn

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!

Hi Apoptosis, B220 isn't exactly another name for CD45R, but rather an isoform of CD45R predominantly found on B cells (hence the B) weighing 220 Kd. As far as I'm aware CD22 (Siglec 2) binds to sialic acids on glycans of glycoproteins and glycolipids. I'm not aware of CD22 binding to B220. As dttom said, even if these molecules do interact this doesn't necessarily mean that B-cells and APC interact through cell to cell contact. Usually, cell to cell interaction involves many receptors and ligands interacting. Hope that helps!