A more comprehensive analysis by liquid chromatography tandem mass spectrometry (LC-MS/MS) revealed a number of other plasma components including complement factors, apolipoproteins and alpha-1-antitrypsin (AAT) that are present in plasma during AHI ( Table S1 ). The factors found to be elevated in plasma during AHI included a C-terminal peptide derived from AAT, residues 377-396, referred to as VIRIP (Table S1 and Fig. 4A ), which was shown to inhibit HIV-1 entry into host cells by targeting the gp41 fusion peptide [10] . Seven plasma panels from HIV-infected individuals and five panels from uninfected controls were evaluated for the presence of VIRIP by tandem mass spectrometry in a semi-quantitative fashion. Ion counts detected for the precursor ion representing the expected molecular mass of VIRIP were correlated to viremia, and revealed an elevation of VIRIP coincident with and after the initial increase in viremia in two of the seven plasma donor panels from infected individuals, but none in the five controls (Fig. 4B) . A semi-quantitative titration of VIRIP peptide by mass spectrometry indicated that the amount detected corresponds to an estimated value of 0.1-0.3mM of VIRIP in plasma at peak concentrations (Fig. 4C ). Considering the sample loss during the isolation of VIRIP peptide from plasma, the effective VIRIP concentration will likely be in the range of low mg/ml, which is approximating the IC 50 value at which VIRIP interferes with HIV-1 entry [10] (see also below).