A method for the absolute quantification of an MHC peptide ligand on a cell, said method comprising a) preparing cells presenting said MHC peptide ligand from a biological sample selected from a tissue sample, a blood sample, a tumor sample, or a sample of an infected tissue comprising said cells, comprising enzymatic digestion of tissues, and/or cellular lysis, b) determining the cell count of said preparation of step a) comprising counting cell nuclei, photometric DNA-determination, fluorimetric DNA-determination, or quantitative PCR, c) adding a known amount of the MHC peptide ligand and/or peptide-MHC complex to be quantified to said preparation of step a) directly after tissue homogenization ("spiking I"), d) isolating the MHC peptide ligand from said preparation of step c) in order to obtain a peptide eluate, e) adding a known amount of the MHC peptide ligand to be quantified to said peptide eluate ("spiking II") as an internal calibrant, f) performing a mass spectrometry analysis on said MHC peptide ligand in order to generate aa) a signal for the efficiency of the isolation in step d), bb) a signal for the internal calibrant of step e), and cc) a signal for said MHC peptide ligand from said prepared cells of step a), and g) quantifying said MHC peptide ligand based on a comparison of the signals as obtained in step f) with aa) the cell count as obtained, bb) the known amount of said MHC peptide ligand and/or peptide-MHC complex to be quantified as added in step c), and cc) the known amount of MHC peptide ligand to be quantified as added in step e), wherein quantifying comprises calculating a ratio between the signals of the internal calibrant of step e) and of the isolated MHC peptide ligand, and comparing the ratio with an established calibration curve of the MHC peptide ligand, and wherein quantifying further comprises the generation of a peptide specific calibration curve based on a ratio with the internal calibrant used at the same amount, and determining the lowest level of quantification (LLOQ) for said MHC peptide ligand to be quantified, whereby an absolute quantification of the MHC peptide ligand on a cell is achieved for the MHC peptide ligand if the quantified amount is above the LLOQ as determined. a) preparing cells presenting said MHC peptide ligand from a biological sample selected from a tissue sample, a blood sample, a tumor sample, or a sample of an infected tissue comprising said cells, comprising enzymatic digestion of tissues, and/or cellular lysis, b) determining the cell count of said preparation of step a) comprising counting cell nuclei, photometric DNA-determination, fluorimetric DNA-determination, or quantitative PCR, c) adding a known amount of the MHC peptide ligand and/or peptide-MHC complex to be quantified to said preparation of step a) directly after tissue homogenization ("spiking I"), d) isolating the MHC peptide ligand from said preparation of step c) in order to obtain a peptide eluate, e) adding a known amount of the MHC peptide ligand to be quantified to said peptide eluate ("spiking II") as an internal calibrant, f) performing a mass spectrometry analysis on said MHC peptide ligand in order to generate aa) a signal for the efficiency of the isolation in step d), bb) a signal for the internal calibrant of step e), and cc) a signal for said MHC peptide ligand from said prepared cells of step a), and aa) a signal for the efficiency of the isolation in step d), bb) a signal for the internal calibrant of step e), and cc) a signal for said MHC peptide ligand from said prepared cells of step a), and g) quantifying said MHC peptide ligand based on a comparison of the signals as obtained in step f) with aa) the cell count as obtained, bb) the known amount of said MHC peptide ligand and/or peptide-MHC complex to be quantified as added in step c), and cc) the known amount of MHC peptide ligand to be quantified as added in step e), wherein quantifying comprises calculating a ratio between the signals of the internal calibrant of step e) and of the isolated MHC peptide ligand, and comparing the ratio with an established calibration curve of the MHC peptide ligand, and wherein quantifying further comprises the generation of a peptide specific calibration curve based on a ratio with the internal calibrant used at the same amount, and determining the lowest level of quantification (LLOQ) for said MHC peptide ligand to be quantified, whereby an absolute quantification of the MHC peptide ligand on a cell is achieved for the MHC peptide ligand if the quantified amount is above the LLOQ as determined. aa) the cell count as obtained, bb) the known amount of said MHC peptide ligand and/or peptide-MHC complex to be quantified as added in step c), and cc) the known amount of MHC peptide ligand to be quantified as added in step e), wherein quantifying comprises calculating a ratio between the signals of the internal calibrant of step e) and of the isolated MHC peptide ligand, and comparing the ratio with an established calibration curve of the MHC peptide ligand, and wherein quantifying further comprises the generation of a peptide specific calibration curve based on a ratio with the internal calibrant used at the same amount, and determining the lowest level of quantification (LLOQ) for said MHC peptide ligand to be quantified, whereby an absolute quantification of the MHC peptide ligand on a cell is achieved for the MHC peptide ligand if the quantified amount is above the LLOQ as determined.