BROWNSVILLE – Plasma donation centers can be found throughout the Rio Grande Valley. They aren't lacking so-called donors either.

It's 5 a.m. in Downtown Brownsville, the streets are dark and still, already a line of people wraps around a plasma center building. It's just a short walk from the Brownsville Gateway International Bridge.

It draws dozens of donors from across the border.

They've made the early trip into the U.S. to exchange their blood plasma for some quick cash.

The first person in line arrived at 3 a.m. to secure a spot ahead of the weekend rush. Doors open at 6 a.m.

Maria Fernanda Isaias crossed the border for the last 14 years to sell her plasma. She said she's tired of doing it. Her small business back home just isn't enough.

"Sometimes you just need the money," Isaias said. "I have a little business over there and I have to come here early enough so that I can get back and open at least at 9 a.m."

Day breaks over the city. Karla Bernal stands outside the plasma center, bandaged up, waiting for her husband to finish. She just endured a three-hour process to sell her plasma.

She comes from Matamoros at least twice a week for this. She said it's her way of providing for her four children.

"It may be for uniforms, school supplies, milk, diapers – all that," Bernal said.

She has a full-time job at an airbag factory in Mexico.

"I work 50 hours a week over there, 10 hours each day," Bernal said. "When I come here, sometimes I wait in line for 3 hours a day and I make what I would at my job over there in a whole week."

It's not fun. She said the thick needle hurts her arm. The scar tissue makes it even worse. It's the painful reality of what she must do.

Between Bernal and her husband, they can make $115 a day. The plasma center sometimes gives bonuses to regular donors. On those days they can make up to $210.

It's a lifestyle Bernal said, she can't afford to give up.

CHANNEL 5 NEWS visited several plasma centers around Brownsville and met students, teachers and moms, from both sides of the border; they all echoed the same stories. Selling their plasma is their supplemental income.

That’s the case for one teacher.

"The strategy of them coming to an area where there's a weak economy, helps," said Pedro Armando Hernandez-Zapata. "Obviously here in the U.S. $50 is not a lot. You can spend $25, $30 on one meal. So here, it's not a lot, but crossing the Rio Bravo, it can go a long way."

So what is plasma? We went to United Blood Services to meet Regional Director Frank Esparza. There they collect blood donations to help patients at 18 hospitals throughout the Valley.

Esparza tells us plasma is a protein-rich component of blood, especially valuable to trauma, burn and cancer patients and patients with bleeding disorders. Those proteins are also used in the private sector, he said, to develop pharmaceutical drugs.

"Plasma is rich in what we call coagulation proteins and other proteins, but coagulation proteins are what actually cause you to clot and stop the bleeding," Esparza said.

Esparza said that at United Blood Services centers, donors aren't paid cash for their donations and the plasma there isn't used for medications. It goes directly to patients.

"The plasma is going to a transfusion at a hospital and it's not going to go to a manufacturing company to make specialized products or resale for profit," Esparza said. "It's going to go to help a patient – our donations go to help patients here locally."

While United Blood Services continues to turn away donors looking for a cash-out, those people are flocking to plasma centers all across the Valley for that cash.

We discovered this discrete plasma center bus terminal, just a short walk from the Hidalgo-Reynosa International Bridge.

Inside, no attendant, ticket booth, not even posters on the wall to indicate what this building is used for.

Chairs are lined up against the wall. People arrive one after the other and take their seat in that order.

Shari Constantino from Reynosa told CHANNEL 5 NEWS, that's the system in place to secure a seat on a small bus. It will take her and the others to a plasma center in downtown McAllen.

She knows the system well. She crosses the border from Reynosa twice a week to sell her plasma. Doing so, she said, gives her financial flexibility.

"I was able to start buying things, like TV's and other things for the house," Constantino said.

A woman tells us she's doing this behind her husband's back. He doesn't agree with her selling her plasma.

"I tell him it's not really a payment, it's like a bonus, it's like something out of gratitude," she said. "It's not a payment because if it were a payment, you know, plasma is really way more valuable because of all its uses. They benefit a lot more from plasma."

Most of the people we met at the various plasma centers had a basic idea of what their plasma is being used for.

Bernal said it's explained to them in the instructional videos they watch before each donation. She's not worried about the specifics.

"Sometimes I do wonder how much they sell (the plasma) for or if it's to really benefit someone or if it's just for profit," Bernal said. "I don't know, but in my case, it helps me and beyond that, it's enough for me, that it helps me financially, because of my children more than anything."

She believes it's a win-win. These companies get the product they want and people like herself can walk across the border with dollars in their pocket.

The donors all described a similar process. They are screened for any illnesses and general health, fill out medical forms, watch a video, then they get pricked and sit for some hours while their plasma is extracted.

It's a detailed process; however, some admitted there is room for someone to be dishonest if they want to get their hands on some fast cash.

Most also said this is not something they enjoy doing, it's simply what they must do to make ends meet.