This study investigates differential DNA methylation in purified human blood cells and its implications for cell lineage and disease susceptibility. Researchers analyzed DNA methylation patterns in six healthy male blood donors, examining whole blood, peripheral blood mononuclear cells (PBMCs), granulocytes, and seven purified cell populations (CD4+ T cells, CD8+ T cells, CD56+ NK cells, CD19+ B cells, CD14+ monocytes, neutrophils, and eosinophils). They found significant variation in methylation profiles among these cell types, with 22% of 8252 probes showing differential methylation between mononuclear cells and granulocytes. For 85% of genes, at least one probe was differentially methylated, indicating that whole blood methylation results may be misleading. The study highlights that DNA methylation patterns vary significantly between cell lineages, with myeloid cells predominantly showing unmethylated CpG sites and lymphoid cells showing marginal or methylated sites. Functional enrichment analysis revealed that genes involved in immune-related functions were differentially methylated, with B cells showing the most distinct patterns. The study also found that DNA methylation in candidate genes for inflammatory diseases varied between cell types, emphasizing the importance of considering cell lineage when interpreting whole blood methylation data. The results suggest that DNA methylation patterns in whole blood may not accurately reflect the methylation status of individual cell types due to the heterogeneity of blood cell populations. This has important implications for studies on disease susceptibility, as differences in methylation may be influenced by the relative proportions of white blood cell types. The study underscores the need for caution when interpreting whole blood methylation data and highlights the importance of considering cell lineage and population-specific methylation patterns in future research.This study investigates differential DNA methylation in purified human blood cells and its implications for cell lineage and disease susceptibility. Researchers analyzed DNA methylation patterns in six healthy male blood donors, examining whole blood, peripheral blood mononuclear cells (PBMCs), granulocytes, and seven purified cell populations (CD4+ T cells, CD8+ T cells, CD56+ NK cells, CD19+ B cells, CD14+ monocytes, neutrophils, and eosinophils). They found significant variation in methylation profiles among these cell types, with 22% of 8252 probes showing differential methylation between mononuclear cells and granulocytes. For 85% of genes, at least one probe was differentially methylated, indicating that whole blood methylation results may be misleading. The study highlights that DNA methylation patterns vary significantly between cell lineages, with myeloid cells predominantly showing unmethylated CpG sites and lymphoid cells showing marginal or methylated sites. Functional enrichment analysis revealed that genes involved in immune-related functions were differentially methylated, with B cells showing the most distinct patterns. The study also found that DNA methylation in candidate genes for inflammatory diseases varied between cell types, emphasizing the importance of considering cell lineage when interpreting whole blood methylation data. The results suggest that DNA methylation patterns in whole blood may not accurately reflect the methylation status of individual cell types due to the heterogeneity of blood cell populations. This has important implications for studies on disease susceptibility, as differences in methylation may be influenced by the relative proportions of white blood cell types. The study underscores the need for caution when interpreting whole blood methylation data and highlights the importance of considering cell lineage and population-specific methylation patterns in future research.