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Charting a Covid-19 Immune Response

Amid a flurry of press conferences delivering upbeat news, President Trump’s doctors have administered an array of experimental therapies that are typically reserved for the most severe cases of Covid-19. Outside observers were left to puzzle through conflicting messages to determine the seriousness of his condition and how it might inform his treatment plan.

Though Mr. Trump may leave Walter Reed National Military Medical Center tonight to continue his recovery in the White House, the future of his health status is unclear. Physicians have warned that the president remains at a precarious point in his disease course. The coronavirus can be a tricky adversary — and for many people whose cases of Covid-19 are severe, the greatest threat to survival might not be the pathogen itself, but the deadly forces that the body marshals to fight it.

To quash the virus, the immune system unleashes an arsenal of powerful weapons. Sometimes these turn inward and destroy healthy tissues. Combatting this friendly fire has become as crucial a part of the Covid-19 treatment strategy as subduing the virus itself.

Mild and Severe Cases

From the moment the coronavirus enters the body, the immune system mounts a defense, launching a battalion of cells and molecules against the invader.

Most people who are infected with the coronavirus recover, sometimes without ever experiencing symptoms, and do not progress to severe Covid-19. In some cases, the virus may even be brought under control before it has the chance to become established in the body.

Should the virus gain a foothold, it will swiftly infiltrate cells and repeatedly copy itself until levels of the virus, or the viral load, build up. The viral load may even peak before symptoms appear, if they appear at all.

Still, symptoms like fever, cough, congestion and fatigue — all of which have been reported in Mr. Trump — signal that an immune response is underway in the body and may be driving the viral load down. Once the immune system has finished the job, symptoms may abate without medical intervention.

In severe cases, however, the clash between the virus and the immune system rages much longer. Other parts of the body, including those not directly affected by the virus, become collateral damage, prompting serious and potentially life-threatening symptoms.

[For more details on the progression of a typical Covid-19 case, see Charting a Coronavirus Infection.]

Triggering the Immune System

A typical immune response launches its defense in two phases. First, a cadre of fast-acting fighters rushes to the site of infection and attempts to corral the invader. This so-called innate response buys the rest of the immune system time to mount a second, more tailored attack, called the adaptive response, which kicks in about a week later, around the time the first wave begins to wane.

In people with severe disease, however, the immune system appears to botch the timing. The first wave mobilizes too late and must play a frantic game of catch-up that persists even after reinforcements