Answer
The older individuals had memory cells which were capable of making antibodies to bind to the 1918 flu strain. The 1918 flu strain was very similar to the 2009 virus strain, which is why these older individuals' immune systems were able to fend off this 2009 strain.
Work Step by Step
The older individuals had memory cells which were capable of making antibodies to bind to the 1918 flu strain. The 1918 flu strain was very similar to the 2009 virus strain, which is why these older individuals' immune systems were able to fend off this 2009 strain.