21.   The reaction depends on factors like the antigen dosage and the presence or absence of danger signals.

22.   The simplest explanation for these phenomena, she believes, is that an antigen must be accompanied by a danger signal before it will touch off an attack.

23.   Though the country has some economic indicators flashing danger signals, many others show a healthy economy.

24.   Though the country has some economic indicators flashing danger signals, many others point to a healthy economy.

25.   Though the country has some economic indicators flashing danger signals, others point to a healthy economy.

26.   U.S. economic conditions could hardly be better, they say, but the psychological danger signals are starting to flash red.

27.   Yet there are more hopeful signs than danger signals in this engaging series.

28.   And they were taught to look for danger signals including dead animals, craters or scorch marks on the ground.

29.   Authorities asked boats to return to safety and ordered port officials in to hoist danger signals.

30.   However, he said his players have only had the upcoming finals in sight and therefore ignored danger signals about injuries.