age and breeddependent adapted immune responsiveness of poultry to intratrachealadministered pathogenassociated molecular patterns immune modulation of poultry by airborne pathogenassociated molecular patterns pamp was studied white and brown layer chicks were exposed intratracheally during 5 consecutive days at 7 wk of age with escherichia coliderived lipopolysaccharide lps saccharomyces cerevisiaederived 13 betaglucan bgl a combination of both or pbs as a control six weeks later birds received similar or crossover pamp treatments body weight gain feed conversion primary and secondary specific antibody responses to model antigens and natural antibody levels were measured in general bgl enhanced but lps exposure decreased primary immune responses at 7 wk of age whereas both pampenhanced secondary immune responses but decreased primary immune responses at 13 wk of age body weight gain and feed conversion at both ages were negatively affected by lps especially in white birds but not by bgl pathogenassociated molecular patterns exposure at 7 wk of age also affected ab responses at 13 wk of age birds exposed to a combination of lps bgl at 7 wk of age had significantly lower secondary total and igg ab responses at 13 wk of age birds from both breeds showed enhanced bw gain after exposure to lps at 13 wk of age when initially challenged at 7 wk of age with lps bgl or a combined challenge with both pathogenassociated molecular patterns exposure at 7 wk of age affected humoral immunity and bw gain at 13 wk of age in a positive bgl or negative lps fashion repeated exposure to pamp did not affect ab responses but crossover exposure to pamp in general enhanced ab responses body weight gain was positively affected by repeated exposure but not by crossover exposure suggesting adaptation of the birds to early pamp exposure our findings suggest that sensitivity of poultry for immune modulation by airborne pamp differs between ages is breeddependent and is not irreversible of nature in addition our data suggest different adaptation to hygienic conditions both with respect to immune reactivity and bw gain