Question about MHC II vs. MHC I

[deleted647690]

BR biology describes macrophages as cells which present an antigen via an MHC I protein so that cytotoxic T cells can recognize and kill the invading cells (nonspecific immune response). In one of the khan academy immunology videos, it says that macrophages bind an invading molecule and then present an antigen via MHC II proteins, not MHC I (specific response). Both of these are correct, right?

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[neurodoc]

I've not read BR, but the way you present the two statements I'd have to say that KA's statement is more generally accurate. All nucleated cells, including macrophages, express MHC I. If the cell is invaded by a virus, undergoes malignant transformation, or for some other reason starts producing abnormal proteins, these are broken down into fragments and are attached to MHC I and transported to the cell membrane. Activated effector cytotoxic T-cells with corresponding receptors then can bind to the cell and destroy it by various means. That's the usual way cells bind antigen to MHC I, and it doesn't involve phagocytosis. The phagocytosis of extracellular antigens and their internal processing and cell-surface presentation of these antigens by specialized antigen presenting cells (APC's) mostly involves binding to MHC II. Dendritic cells, however, are known to be able, in vivo, to phagocytize exogenous pathogens and present them attached to MHC I so as to activate cytotoxic T-cells. They do this by a slightly different pathway than the one used by cells to present endogenous pathogenic antigens, in a process called "cross-presentation." Macrophages usually do not do this in vivo but they have been observed to do so in tissue culture experiments. This is just my limited understanding of this stuff, and if I've gotten something wrong I'm sure someone will chime in.

 

[Pono_0001]

@neurodoc has it right. I'm looking at the BR book on that right now. I would say BR is wrong as it gives the impressive that MHC I is only on macrophage. MHC II is only on APC (e.g. macrophage, NK, dentritic cells etc), and MHC I is on all nucleated cells. I think this is the main point that you should know, and it's a point that BR misses.

So Macrophage have can present antigen via both MHC I and MHC II. But the main difference between MHC I and MHC II is this: MHC II display peptides that are from extracellular space and MHC I present antigens from intracellular space (or from extracellular space via cross-presentation in the case of DC or other pAPC, but this may be out of the scope of MCAT). And this makes sense when you think what their respective activation. Activating helper T cells by MHC II presenting antigens from extracellular space lead to humoral mediated response that's not effective for cells who are already affected, and this is where MHC I comes in. And then there's cross presentation too, find an immunology book that will go into detail on the importance of cross presentation of extracellular antigens via MHC I in activating naive CD8+ T cells. but it's still an active area of research and out of the scope of MCAT