Sickle cell trait sidelines football player: What is it?

(CBS/AP) A sickle cell trait will keep Pittsburgh Steelers safety Ryan Clark out of Sunday's AFC Wild Card game in Denver. The trait makes it dangerous for the 32-year-old to play in the high altitudes at Denver's Mile High Stadium.

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Coach Mike Tomlin said Tuesday having Clark suit up is not worth the risk. Clark lost his spleen, gall bladder and 30 pounds after the condition was triggered when playing in Denver in 2007. Clark was also held out of Pittsburgh's last trip to Denver in 2009.

Just what exactly is a sickle cell trait? People who inherit one sickle cell gene and one normal gene have sickle cell trait. That's compared to people who inherit one sickle cell gene from each parent, who may get sickle cell anemia, the most severe form of sickle cell disease.

People with the trait usually do not have the symptoms of sickle cell disease, but they might experience complications of the disease, such as pain crises, which occur when the sickled cell becomes stuck in the blood vessel, causing painful swelling. In rare cases, people with the trait can face harm when they're dehydrated, in an atmosphere with increased pressure - like when scuba diving - or when they are in areas with high altitude and low oxygen.

Athletes with sickle cell trait are more likely to experience heat stroke and muscle breakdown when exercising in very high or low temperatures, according to the CDC. To prevent illness while playing competitive sports, the CDC recommends people with sickle cell trait set their own pace and slowly build workout intensity, rest often between activities, and drink plenty of water.

The CDC has more on sickle cell trait.