* Dozens of AIDS experts feared killed in downed airliner
* Joep Lange, dedicated AIDS fighter, mourned by peers
* UNAIDS director says AIDS response suffers great loss
* WHO mourns loss of staffer with "passion for public health" (Adds fresh comment, details)
By Sonali Paul and Robin Emmott
MELBOURNE/AMSTERDAM, July 18 (Reuters) - The world of AIDS research was in shock on Friday after dozens of leading HIV experts were feared killed when a Malaysian plane was shot down over Ukraine, in a setback for the global campaign against the disease.
Among them was Joep Lange, who researched the condition for more than 30 years and was considered a giant in the field, admired for his tireless advocacy for access to affordable AIDS drugs for HIV positive patients living in poor countries.
"Global health and the AIDS response have lost one of their great leaders," Peter Piot, director of the London School of Hygiene & Tropical Medicine and a former executive director of UNAIDS, told Reuters in London.
"Joep Lange was one of the most creative AIDS researchers, a humanist, and tireless organiser, dedicated to his patients and to defeating AIDS in the poorest countries."
The United Nations AIDS program, UNAIDS, said it feared "some of the finest academics, health-care workers and activists in the AIDS response may have perished" on the plane.
"Professor Lange was a leading light in the field since the early days of HIV and worked unceasingly to widen access to antiretroviral medicines around the world," it said.
As many as 100 people heading to the AIDS 2014 conference in Melbourne were on the doomed flight, Fairfax Media reported, including Lange, a former president of the International AIDS Society (IAS) which organises the event.
"The UNAIDS family is in deep shock...The deaths of so many committed people working against HIV will be a great loss for the AIDS response," said Michel Sidibe, executive director of UNAIDS.
The conference, due to start on Sunday, features former U.S. President Bill Clinton among its keynote speakers and is expecting around 12,000 participants.
The IAS said it was still working with authorities to confirm the number of delegates on the flight and would go ahead with the conference as planned.
Peers paid tribute to Lange, a Dutch professor of medicine at the Academic Medical Center at the University of Amsterdam.
The Malaysian Airlines Boeing 777 was en route from Amsterdam to Kuala Lumpur when it was shot down on Thursday by a surface-to-air missile in an area of eastern Ukraine where Moscow-backed rebels have been fighting government forces.
Lange pioneered access to key AIDS medicines in poor countries, including combination drugs to control HIV and antiretroviral medicines to prevent transmission of the virus from mothers to their babies.
Robin Weiss, a professor of viral oncology at University College London, compared Lange to Jonathan Mann, a key figure in the early fight against HIV/AIDS who was killed along with his wife and fellow AIDS researcher Mary Lou Clements-Mann on a Swissair flight to Geneva in 1998.
"Not since the loss of Jonathan Mann and his wife...has the HIV/AIDS research community suffered such a great loss," he said.
Weiss added that while the HIV/AIDS movement as a whole may be weakened by the loss of so many activists and specialists, scientific progress would continue.
"It's a very sad loss, but the momentum will carry on," he told Reuters.
Lange's colleague Jaap Goudsmit described his co-worker and friend as a great activist and fighter for the cause.
"He was there from the beginning, from...when we were seeing young people dying very fast and no one knew why," he said.
"He was a fighter for getting treatment to everyone who needed it and as early as possible to lower the spread of infection. His clinical contribution was enormous."
The World Health Organization (WHO) said media spokesman Glenn Thomas was among those on board Flight MH17.
Gregory Hartl, a WHO spokesman, said Thomas had been with the organisation for more than a decade and "will be remembered for his ready laugh and passion for public health."
"He will be greatly missed by those who had the opportunity to know him and work with him. He leaves behind his partner Claudio and his twin sister Tracey."
Thomas, a British national, was in charge of promoting the WHO's report issued last week that said five key groups including gay men had stubbornly high rates of HIV. (Writing and reporting by Kate Kelland in LONDON and Stephanie Nebehay in GENEVA, additional reporting by and Jane Wardell and Lincoln Feast in SYDNEY.; Editing by Mike Collett-White and Stephen Coates)