A Critical Rectal Temperature Defines Sex-Specific Leukocyte and Neutrophil Activation During Prolonged Passive Heat Exposure in Young Adults

BackgroundProlonged heat exposure disrupts immune homeostasis and can precipitate acute systemic inflammation. However, the core temperature threshold that triggers sex-specific leukocyte and neutrophil activation during passive heat stress remains undefined. MethodsWe studied 52 males and 58 females exposed to wet-bulb temperatures of 32-35 {degrees}C. Rectal temperature (Trec) was continuously monitored, and blood samples were collected at 0.5 {degrees}C increments up to 38.6 {degrees}C. Leukocyte and neutrophil counts were modeled using quadratic and segmented regression to identify inflection points of immune activation. ResultsBoth leukocytes and neutrophils increased nonlinearly with rising Trec (p < 0.05). A critical Trec of 38.1 {degrees}C--closely aligns with the 38.0 {degrees}C occupational core temperature limits--marked an inflection in leukocyte responses: below this point, females showed steeper increases; whereas above it, males showed accelerated activation. Neutrophils demonstrated consistently greater mobilization in males across the entire temperature range (36.9-38.6 {degrees}C). ConclusionsA distinct core temperature threshold ([~]38.1 {degrees}C) governs immune cell activation and reveals sex-dependent response patterns. This finding provides an immunological rationale for current occupational het limits and highlights the importance of integrating sex-specific considerations into protective guidelines under extreme heat. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=140 SRC="FIGDIR/small/681783v1_ufig1.gif" ALT="Figure 1"> View larger version (22K): org.highwire.dtl.DTLVardef@9620dorg.highwire.dtl.DTLVardef@47beborg.highwire.dtl.DTLVardef@6396daorg.highwire.dtl.DTLVardef@44c890_HPS_FORMAT_FIGEXP M_FIG C_FIG