Which of these molecules is present determines what types of cells the T cell can bind to.
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CD8+ T cells bind epitopes that are part of class I histocompatibility molecules. Almost all the cells of the body express class I molecules.
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CD4+ T cells bind epitopes that are part of class II histocompatibility molecules. Only specialized antigen-presenting cells express class II molecules. These include:dendritic cells, phagocytic cells like macrophages and B cells!
CD8+ T cells
The best understood CD8+ T cells are cytotoxic T lymphocytes (CTLs). They secrete molecules that destroy the cell to which they have bound. This is a very useful function if the target cell is infected with a virus because the cell is usually destroyed before it can release a fresh crop of virus particles that leave the cell (often killing it in the process) and spread to new target cells.
Class I and class II molecules present antigen fragments to different subsets of T cells.
Most of the T cells of the body belong to one of two distinct subsets: CD4+ or CD8+. CD4 and CD8 are surface glycoproteins. Both CD4+ and CD8+ T cells have an antigen receptor (TCR) that "sees" a complex histocompatibility molecule. However, neither type can be activated by simply binding its complementary epitope. An additional molecular interaction must take place.
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The CD8 molecules on CD8+ T cells bind to a site found only on class I histocompatibility molecules.
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The CD4 molecules on CD4+ T cells bind to a site found only on class II histocompatibility molecules.
The CD8+ T Cell/Class I Interaction
Because of the need for CD8 to bind to a receptor site found only on class I histocompatibility molecules, CD8+ T cells are only able to respond to antigens presented by class I molecules. Most CD8+ T cells are cytotoxic T cells (CTLs). They contain the machinery for destroying cells whose class I epitope they recognize.
An example:
Every time you get a viral infection, say influenza (flu), the virus invades certain cells of your body. Once inside, the virus subverts the metabolism of the cell to make more virus molecules. This involves synthesizing molecules encoded by the viral genome.
While in transit from their old home to their new, the virus works inside cells safe from any antibodies. But early in their intracellular life, infected cells display fragments of the viral proteins being synthesized in the cytoplasm in their surface class I molecules.
Any cytotoxic T cells specific for that antigen will bind to the infected cell and often will be able to destroy it before it can release the virus.
In general, the function of the body's CD8+ T cells is to monitor all the cells of the body ready to destroy any that express foreign antigen fragments in their class I molecules.
The newly isolated HIV-3 virus is especially dangerous as it affects CD8+ as well as CD4+ cells. This means that there are no governing cells and the infected T cells are free to develop. New vaccines that are being perfected will have no affect on these infected CD8+ cells due to their structural specificity.
