I was just wondering what the difference is between an antigen and a MHC marker and their functions, because in the N.O.B textbook they use these terms interchangeably. Furthermore, what is the difference between MHC class 1 and class 2 markers, and their function?
To my knowledge, MHC, or major histocompatibility complex, is the gene which codes for the MHC markers, which are presented on the surface of the cell (protruding from the cell's plasma membrane). The function of an MHC marker is to present the antigen in order for the immune cells to recognise the cell as 'self' or 'non-self'. Class 1 MHC markers, which are situated on all cells except red blood cells, present 'self' antigens, while Class 2 MHC markers present 'non-self' antigens, thus they are only found on specific immune cells, such as T cells, B cells, and some macrophages. I think that when an infectious pathogen invades an organism, the foreign antigen enters these immune cells via endocytosis, and through antigen presentation, they process and place the foreign antigen on MHC class 2 markers (on that note, would T cells, B cells, and some macrophages be considered as APCs [antigen presenting cells]?).
Is this correct? Is it appropriate to imagine the MHC marker as a key-ring, and the antigen as they key which initiates an immune response? (or for this instance, opening a door). Also, if erythrocytes, or red blood cells, have no MHC markers, how can they be able to present A and B antigens? also, how would they even produce an antigen for that matter seeing that they lack a nucleus (transcription and translation wouldn't occur right?).
Finally, what modes of prevention from the second line of defence apply to different pathogens? For instance, how would macrophages engulfing a virus via phagocytosis be effective (wouldn't this just further spread the viral infection)? Do complement proteins apply to all pathogens, or just bacteria, like interferons only apply to viral infections? In the N.O.B, they tend to use micro-organisms quite frequently, making myself disassociate viral and prion infections from most modes of prevention of the second line of defence, but I'm just looking for clarification... Also, how do Natural Killer cells function? the function of Nk cells isn't really explained either in the N.O.B 
. Um and how exactly to interferons work? I'm pretty sure they are secreted by cells infected by a virus, warning surrounding cells to build viral resistance via the production of antiviral proteins - just wanted to check if this is correct...
THANK YOU 
1. Antigens are foreign materials that stimulate the production of antibodies (hence antigen is antibody-generating). An MHC molecule, however, is a protein complex that presents antigens to T-cells. B-cells can bind to free antigen and Bob's your uncle, but T-cells need some extra help and can only recognise antigens when they're presented in the context of (i.e. attached to) an MHC molecule.
2. MHC class I activate cytotoxic T-cells, MHC class II activate helper T-cells. Otherwise there's very little different about them.
3. Assuming that information is correct, then yes they are all APCs. B-cells and macrophages definitely are, as well as dendritic cells. Didn't know about the T-cells though.
4. Not the best analogy but I guess so?
5. Antigens don't need to be presented to be antigens. A and B (and H as well, but we won't go into that) antigens are just glycoproteins that sit on the surface of RBCs. What you've got to appreciate is that antigens aren't specially designed to specify self or non-self. They're just normal proteins or carbs that have other functions that so happen to generate an immune response when introduced as a foreign substance.
6. There are some differences though I'm sure you could google this. The bottom line is that it's complex and involves a lot of crossover. Generally though, complement tends to be a bit more useful in bacterial infectinos, and IFN more useful in viral. Though it's not as though there isn't cross-over here either. You're right to point out about macrophages acting as a source of spread for viruses though. In some cases they do (and for some bacteria too like Tuberculosis for example), but in others they successfully kill the pathogen thereby preventing their spread.
7. That's a VCE-acceptable definition of interferons, but they are more complicated than that
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