Axial chirality

Have a look at the structures (a) and (b). They are the stereoisomers of laballenic acid, with (a) is naturally occurring in plants of the Lamiaceae family. What kind of stereoisomers are they?

(a)	(b)

(−)-laballenic acid (trivial) (5M)-octadeca-5,6-dienoic acid (substitutive, PIN) (5Ra)-octadeca-5,6-dienoic acid (substitutive) (+)-laballenic acid (trivial) (5P)-octadeca-5,6-dienoic acid (substitutive, PIN) (5Sa)-octadeca-5,6-dienoic acid (substitutive)

If there was just one double bond in the middle of the molecule, we’ll be dealing with cis/trans isomerism. But we have two cumulative double bonds, which makes our molecules chiral, even though there are no chiral atoms. Why?