Abbott Laboratories Inc. added several new warnings to the label for its rheumatoid arthritis drug Humira, according to a company letter released on Monday.

Abbott, in a letter to doctors posted on the Food and Drug Administration Web site, said the FDA and the company had received “rare” reports of hypersensitivity and blood cell deficiencies, including aplastic anemia and cytopenia.

When used in combination with another arthritis drug called anakinra, some serious infections were also reported. Amgen Inc. markets anakinra under the brand name Kineret.

The letter, dated Nov. 5, said while the relationship between Humira and reported blood problems “remains unclear,” patients should seek medical attention if they show signs of fever, bruising or other symptoms while taking the drug.

Abbott spokesman Jim Bozikis said the company revised the label in June but sent doctors the letter this month to let them know the company’s language was “consistent” with warnings from other drugs in the same class.

Rival arthritis drugs, Amgen’s Enbrel and Johnson & Johnson’s Remicade also carry various warnings, including ones for blood disorders.

All three prescription drugs work by blocking an inflammation-causing protein called tumor necrosis factor. The drugs also carry warnings that they may raise the risk of contracting tuberculosis.

Despite those rare problems, the medicines are enjoying strong revenue growth because of their ability to control symptoms of rheumatoid arthritis - the less common and hereditary form of arthritis in which the immune system attacks joints.

About 75,000 patients worldwide have been treated with Humira since 2003. The company projects Humira sales of $800 million this year, and $1.2 billion in 2005, Bozikis said.

Abbott shares closed up 51 cents, or 1.15 percent, to $44.96 on Monday on the New York Stock Exchange. Shares of Amgen closed down 61 cents, or about 1 percent, to $60.05 on the Nasdaq.

The letter and revised label can be found online at http://www.fda.gov/medwatch/SAFETY/2004/safety04.htm