Although this sounds like a good beginner's question I have found no corresponding textbook passage.

It should make sense for antigen presenting cells - APCs - to present only antigen that can be used for recognition. In other words: why should APC present antigens to T-cells if these cannot use such kind of antigen to recognize and attack cells because those antigens are part of inner structural proteins that do not show on the external surface?

If an APC does not have a receptor for an antigen because the antigen is a inner structural of viruses or bacteria why should it present such antigen for the purpose of recognition?

The first answer given to me is a good one as it differentiates MHCI from MHCII presentation. Any authority or textbook passage on my understanding that B-cells take up, via B-cell-receptor, inner structural antigen that comes as cell debris, thereby in need to be activated by t4/t-helper-cells which themselves thus need to be presented inner structural antigen? But then, the B-cell receptor in fact is a receptor in terms of my question. Which interestingly makes it seem a possibility that macrophages and dendritic cells have receptors that do not recognize any inner structural protein that comes as cell debris.

Having gone through and read Kuby, Janeway, Roitt on immunology, my understanding is that not each and every antigen of a virus will find its way to the MHC complex on the cell membrane of antigen presenting cells. If that is correct, how could eukaryotic cells possibly differentiate between antigen of viruses they present and those they would not? Do they present not "all sorts" of antigen but only those they have receptors for?

My question is not restricted to APC, but includes "normal" cells of the body that become infected and are not part of the immune system. For instance, CoV-19 infects pneumocytes that express the ACE2-receptor. It seems CoV-19 infected body cells do present only the spike protein on MHC. Is that correct?

My question does not refer to the diffent pathways leading to MHCI or II, does not refer to the difference between exogeneous and endogeneous antigens. According to comment see below my question should be restricted to exogeneous antigens presented on MHCII. However, if uptake is by receptor or not, if there is receptor-mediated endocytoses or not does not correlate to MHCI and MHCII as alternative pathways of presentation. Interestingly, APC cells do use receptors (macrophages and dendritic cells use PAMPS, B-cells the BC-Receptor.

So any antigen that cannot be considered antigen to a receptor and becomes presented would answer my question to the negativ. On the other hand, there do exist proteins made form viral RNA that will not become part of the outer membrane of the virus and will never encounter any receptor or contact antigen of the body as they are part of the inner structure of the virus.

Are the latter being presented on MHC? Give me one example, please.


"On MHC II, do cells present only antigen they have receptors for?"

Thank you for boiling your question down to its essence. The answer is no. Professional antigen-presenting cells (APCs) such as macrophages, dendritic cells, B cells, etc., which present peptides in the context of the major histocompatibility complex (MHC) Class II to CD4+ Thelper cells, initially obtain the source proteins through non-specific phagocytosis. Receptor binding doesn't necessarily have anything to do with it, unless you're looking specifically at B cells and antigen's binding to the B cell receptor (BCR) before being endocytosed.

MHC II presentation

From immunology.org

Where you might be confused about receptors is that only some of the peptides generated by cathepsin-mediated proteolysis in the phagocytic endosome have high-enough structural affinity to bind the MHC II receptor by displacing the CLIP fragment and being exported to the cell surface.


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