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The Red Cross is asking Southern Californians who have survived COVID-19 to donate blood as part of a nationwide effort to help seriously ill patients.

The nonprofit’s local chapter on Sunday called for donors who had a confirmed COVID-19 diagnosis but have fully recovered. Their plasma—the liquid part of the blood—have antibodies that may help current patients fight the virus.

Historically, so-called convalescent plasma has been used to treat infections such as polio, hepatitis, influenza and Ebola when there were no treatments available. 

In the case of COVID-19 patients, experts have emphasized that the effectiveness of convalescent plasma is still being investigated.

But some success has been reported in a few patients in China, and there have been anecdotal reports of positive outcomes in the U.S., according to Dr. Ellen Klapper, medical director of transfusion medicine at Cedars-Sinai.  

With the general safety of plasma transfusions and no approved treatments for COVID-19, the Food and Drug Administration began allowing the investigational use of convalescent plasma on some patients.

A similar effort by a group of doctors and researchers from more than 50 institutions across the U.S., including Johns Hopkins University and University of California Los Angeles, connects potential donors to health care providers and patients. 

The FDA, on the other hand, has tapped the Red Cross to help facilitate the collection and distribution of plasma to patients in need. 

“Nearly all U.S. blood centers, including the Red Cross, are mobilizing to assist with the collection of COVID-19 convalescent plasma,” organization spokeswoman Christine Welch said.

Those interested in giving their plasma, which the Red Cross says involves just a few minutes longer than donating blood, must meet a specific set of requirements such as being symptom-free for at least two weeks. 

Potential donors must fill out a form online.

Red Cross will contact volunteers, and information of those eligible will be sent to a donation site so they can schedule an appointment.