By Helen Briggs BBC News Online science reporter

Cloned calves that produce human antibodies in their blood have been born in the United States.

The four cows have extra DNA which contains the genes for the part of the human immune system that makes disease-fighting antibodies.

Scientists believe cows could eventually be used to produce medicines to treat multiple sclerosis, infections and even cancer.

The cows have a human immune system

Yann Echelard, animal cloning expert

Human antibodies have been produced in mice before, but cattle are bigger and make more of them.

The work was carried out by researchers in the United States led by animal cloning pioneer James Robl.

The former Professor at the University of Massachusetts was the first to clone a transgenic cow in 1998.

New drugs

He is now President of Hematech in Westport, Connecticut, a biotechnology firm set up to manufacture human antibodies in cattle.

Dr Robl told BBC News Online: "The antibodies that we produce consist of a large collection of different types that will be particularly useful for killing infectious disease agents."

Dr James Robl (centre)

He says they are quite optimistic about the chance of cloned cows providing a new source of human antibody therapeutics.

"We believe that by successfully transferring the antibody genes into cows we have overcome one of the most difficult challenges in the project."

Yann Echelard, an animal cloning expert at Genzyme Transgenics Corporation, Massachusetts, says the cloned cows could eventually have important medical applications.

"The cows have a human immune system," he told BBC News Online.

"You can immunise them, collect their blood, get the antibodies out, purify them and give them to patients."

'First step'

Antibodies are used for the treatment of many human diseases including immune deficiencies, infectious diseases, and autoimmune disorders.

They have to be extracted from blood donations and are in short supply.

But several hurdles must be overcome before human antibodies from cows could reach the hospital.

Scientists have to find a way to purify the human antibodies and make sure they are free of harmful viruses.

"This is an important step but it is a first step in a process that will go on for years before there is a medicine available to the general public," said Dr Echelard.

Artificial chromosome

The existence of the four cloned cattle is revealed in the journal Nature Biotechnology.

The first calf, Yoon, was born last November. She was named after a graduate student who spent many nights looking after the animals. About 20 similar cloned cows have been born since then.

The calves are known as transchromosomic. Unlike other cows they have an extra synthetic chromosome - one of the bundles of DNA and protein that carries genetic information.

An artificial chromosome has been put into the animals to carry human immune system genes.

Main image courtesy Hematech, LLC and Kirin Brewing Co.