In order to analyse the factors causing the variation, we performed a normed PCA with 32 traits (Tables 1 and 2 ). For cellmediated adaptive IR, we included only those traits related to cytokine production and lymphocyte proliferation after PMAionomycin stimulation. For innate ITs, we included 10 FACScharacterized cell subtypes including the percentages of B lymphocytes (IgM + ), cd T lymphocytes (TCRcd + ), three subsets of ab T lymphocytes (CD4 + CD8 + , CD4 -CD8 + and CD4 + CD8 -), NK cells (CD16 + CD2 + ) and three monocyte subsets (CD16 + CD172A + , CD16 + MHCII + , MHCII + CD172A + ). We excluded from the analysis four haematological traits not directly involved in immunity: red blood cell count (RBC), hematocrit (HT), red blood cell distribution width (RDW) and platelet count (PLT). The percentage of variance (inertia) explained by the first five components was over 50% (Figure 2A ). Each of these five components explained more than 5% of the total variance and the first two components accounted for 16.4 and 10.8% of the total variance, respectively. Taking into account the 32 components from PCA, multivariate normal mixture modelling and modelbased clustering (see Materials and Methods) were used to identify clusters of ITs. The highest Bayesian Information Criterion (BIC) was obtained using the diagonal model with variable shape and variable variance (VVI in pink on Figure 2B ) and K = 3 (first factorial plan on Figure 2C ). No parameter was located near the correlation circle indicating that the phenotypic correlations between ITs are globally weak. A first cluster (K1 in blue on Figure 2C ) groups together four hemogram-derived cell counts: white blood cell count (WBC), lymphocyte count (LYM), monocyte count (MON) and neutrophil count (NEU). This cluster is representative of total cell number traits despite the eosinophil count (EOS) not being included. A second cluster (K2 in green on Figure 2C ) groups together all the traits related to FACScharacterized leukocyte subpopulations (expressed as the percentage of cells with one or two surface antigens) except cd T lymphocytes (TCRcd + ), with a cell response parameter (IL10-PMAIONO) and the seric level of haptoglobin (HAPT). This cluster can be considered as representative of the leukocyte subsets. A third cluster (K3 in red on Figure 2C ) includes all other cell response traits, one hemogram-derived cell count (EOS) and one FACS-characterized leukocyte subpopulation. Note that K1 and K2 related traits, which mainly correspond to cell subsets and explain around 25% of the phenotypic variance, show clear clustering ( Figure 2C ). These traits are grouped on the first PCA axis and separated on the second axis. Traits belonging to K3, representative of cell activity (cytokine production, phagocytosis and antibody production), are more spread out on the other axes (data not shown). Interestingly, cluster analysis did not highlight any cluster of innate or adaptive ITs.