T cells surveil the RNA of bodily cells, looking for molecular patterns that are characteristic of viruses. Like telltale fingerprints, viral manipulation of RNA leaves traces that T cells can sense. Some savvy viruses wipe away giveaway fragments. Others place decoys that confound the immune system. Adept practitioners of stealth, viruses are especially difficult to detect.
There are 4 types of T cells: killer (cytotoxic), helper, memory, and regulatory.
Killer T cells destroy virally infected cells and tumor cells.
Helper T assist both B cells and killer T in their jobs. Helper T cells also play a role in the education of killer T. Helper T cells are the invasion target of HIV, the AIDS virus.
Memory T cells are antigen-specific T historians: T cells that persist after an infection; old soldiers that remember the war. If re-exposed to their cognate antigen, memory T proliferate to T cells that can fight again.
Regulatory T are the maintainers of immunological tolerance: keeping the immune system from attacking self-antigens. (Regulatory T cells were formerly known as suppressor T cells.) Regulatory T cells shut down T cell immune response once an infection is defeated. Regulatory T also suppress auto-reactive T cells that fail T cell training in the thymus.