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 ?)
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.
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.