The finding may hold out the prospect of better treatments and perhaps eventually a cure, the researchers said.
Estimates suggest that nearly two-thirds of people aged 80 years or older are affected by AMD to some degree, with more than one in ten left blind by the disease.
Earlier, scientists identified a number of other genes or genetic loci (regions of the genome), which affect a person's susceptibility to the disease.
And now, researchers at the University of Southampton have shown that a particular variant of the gene SERPING1, carried by just under a quarter of the population, may provide protection against the disease.
In collaboration with the team from the University of Iowa, the researchers found evidence of proteins expressed by SERPING1 in the retina and the choroid layer (the vascular layer next to the retina), the two areas affected by AMD.
The proteins are found to partly regulate the body's innate immune system known as the complement system.
The researchers deduced that the complement system is malfunctioning, attacking the retina and choroid layer.
"It seems counterintuitive that a generalised innate immunity defence system should result in a localised disease of the eye in the elderly," the Lancet quoted Professor Andrew Lotery from the University of Southampton, corresponding author on the study, as saying.
He added: "However, it is becoming increasingly clear from research that this is the case. Previous AMD genes have already implicated the "alternate" complement pathway, and our paper shows that the 'classical' complement pathway is also involved in this process."
The research is published in the latest issue of the Lancet.