These are some of the tests used to measure memory T-cell responses. While the presence of memory T-cells specific for a particular infection (e.g. CMV) is often associated with protection from illness, this is not always the case. Researchers are still working to understand which tests can provide the most accurate measure of protection against a given disease.
A test that introduces the antigen into a patch of skin on the arm. Swelling typically reflects the presence of a memory response to that particular antigen (as is seen with the TB skin test). DTH has been used for decades but is not very sensitive or easy to standardize -- measuring the swelling only provides a very rough gauge of the strength of the memory response.
A laboratory test that measures the ability of memory T-cells (almost always CD4 cells) to copy themselves (proliferate) when exposed to a given antigen. Newly-made cells are counted using a specially-labeled DNA ingredient called thymidine which is incorporated into the new cells during the copying process. The test takes six days to produce a read-out and can be difficult to analyze. Results are usually given as a "stimulation index," which subtracts background proliferation (the number of cells that copied themselves without being stimulated with any antigen).
A lab test that measures the ability of memory T-cells (CD4, CD8, or both) to make cytokines when exposed to a given antigen. T-cells are first exposed to antigen, then 6-24 hours later antibodies that bind to a specific cytokine (most often interferongamma, but others such as IL-2 or IL-10 can also be tested for) are introduced. The cells are chemically treated so that any antibodies bound to cytokine-producing cells are stained blue and can be counted. They're called "spot-forming cells" or SFC for short. As with background proliferation, background cytokine production can be a problem and must be subtracted to get an idea of how many T-cells were specifically responding to the antigen.
Another lab test that measures the ability of memory T-cells (CD4, CD8 or both) to make cytokines when exposed to a given antigen. Unlike ELISpot, this test employs a substance that traps the cytokine within the T-cell. This allows the precise type of T-cell that is making a given cytokine to be more easily identified.