A particular type of blue-green algae may increase the production of disease-fighting cells.

Extracts from algae have long been touted as possible therapies for conditions as varied as diabetes, anaemia, liver disease and ulcers.

Scientists have consistently attacked the "research" on which these claims are made - and concerns have been raised about possible harmful side-effects such as gastroenteritis and hepatitis caused by some varieties of algae.

However, a small study carried out by the University of Californa Davis School of Medicine and Medical Centre suggests that the chemicals in spirulina, a variety of algae, could have a marked effect on the immune system.

Blood samples were collected from 12 healthy volunteers, and various blood cells associated with the immune response.

These were incubated in the laboratory with dilutions of spirulina, measuring any increase in chemicals linked to immune response.

Large increase

The boost to one in particular, interferon-gamma, was sizeable, a 13.6 fold increase.

The levels of interleuin-4 and interleukin-1 beta were more than tripled.

Professor Eric Gershwin, chief of the Division of Rheumatology, Allergy and Clinical Immunology at UC Davis, said: "Together, increases in these cytokines suggest that spirulina is a strong proponent for protecting against intracellular pathogens and parasites."

He called for more research into the effectiveness of the algae extract.

Other studies in animals seem to support the idea that spirulina can help the body fight disease.

In cats, it increased the ability of specialised cells to engulf harmful bacteria, and in chickens, antibody responses were increased.

While analysis of spirulina has shown that it does not contain a harmful toxin called microcystin, other blue-green algae has found to be frequently contaminated.

In May 2000, the Oregon Department of Health surveyed 87 samples of non-spirulina blue green algae, finding that 63 had microcystin levels above the regulatory limit.