Immune system up-regulation in securely attached infants during early childhood.

Early caregiving relationships shape the coordination of stress and immune systems, yet their biological correlates in early infancy remain insufficiently understood. This study examined whether attachment relationships are associated with mucosal immune function and stress physiology. Thirty-five infants (mean age = 16.6 months) were classified as securely or insecurely attached using the Strange Situation Procedure. Salivary secretory IgA (SIgA) was collected in the morning and afternoon at two time points, and cumulative cortisol was quantified from hair samples. Securely attached infants showed higher morning SIgA concentrations and more stable intra-day immune profiles compared with insecurely attached children. No group-level differences were observed for cumulative cortisol, but immune-endocrine associations revealed that higher cortisol was linked to lower morning SIgA and greater intra-day fluctuation. Bayesian regression models supported consistent directional effects, and machine-learning analyses confirmed that SIgA-based features accurately predict attachment type. Our findings support the idea that secure attachment fosters stable coordination between immune and endocrine systems during a critical stage of early development. These effects could be readily captured at very early stages of life, identifying SIgA as a potential biomarker of early socioemotional environments. By integrating behavioral, immunological, and computational approaches, this study provides evidence for the biological embedding of attachment and highlights the potential of non-invasive biomarkers to support early identification of psychosocial vulnerability.