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T-cells recognize the MHC molecules and body's own peptides. When it doesn't, it alarms the immune system. But do T-cells express MHC molecules ? If so, how are they using it? If not, what happens when a virus infects T-cells? (Yes, I am confused about HIV infection mechanism too. They escape from immune response by altering their genes and disrupting the MHC-peptide bonding. I can see that this can work in macrophages. But what is the situation in T-cells ?)

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There are two types of MHC molecules:

All nucleated cells express MHC class 1 proteins. A sample of all proteins produced within the cell are 'sliced' into their constituents which are then presented by MHC-I receptors. This means that if a virus infects a cell, the foreign proteins the cell is induced to make are expressed on the cell membrane by MHC-I. This is important as antibodies can not cross cell membranes, therefore this is the method by which a cell signals that it is infected by a virus.

MHC II are the special molecules that are only expressed in antigen-presenting cells, which I think is what you are referring to. As neither CD4 or CD8 T-cells present antigens, they do not express MHC-II themselves. Instead they have receptors for MHCII, allowing them to interact with antigens presented by other immune cells.

If a virus infects T-cells, the viral proteins are sampled by the MHC-I pathway and presented via MHC-I on the surface of the infected T-Cell. The immune response then progresses as for any other virally infected cell, destruction by CD8 T-Killers that destroy the cell by non-phagocytic means.

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Are you positive CD4 cells don't present antigens? I could have sworn they had the capability to. –  MCM Dec 31 '12 at 22:14
    
@MCM that was off the top of my head, I'll have a read of my notes tomorrow. Happy New Year! –  Rory M Dec 31 '12 at 22:20
    
Are antigen-presenting cells a specific type of cells? I've always been confused about this because the label seems so general. –  shigeta Jan 1 '13 at 15:37
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@shigeta I would say they're a class of cell, defined by their expression of MHC-II. –  Rory M Jan 1 '13 at 22:49
    
I would expect that they do something else besides only presenting antigen - is that their only function? –  shigeta Jan 1 '13 at 23:15
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It would depend on the cell type... dendritic cells, macrophages, B cells can all present antigen, but they also have different functions. For example, macrophages also help to remove pathogens and clean up after an infection. B cells make antibodies etc.

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This question is asking specifically about T cells –  Rory M Oct 26 '13 at 11:32
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