Can someone donate antibodies?

Can someone donate antibodies?

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Consider the following scenario:

  • Person A has been infected with some virus (for example, SARS-CoV-2)
  • Person A has recovered

From this scenario, I assume Person A has managed to develop antibodies for SARS-CoV-2. If so:

  1. For how long, after recovery, would Person A have antibodies for the virus running in their blood?

  2. If Person A donates blood to Person B, would Person B have better responses against the virus, by using the antibodies received along the blood from Person A? (assuming the donation is successful without issues)

  3. Is there any paper / study building upon this idea for combating SARS-CoV-2?


  • I am not expecting exact quantitative answers to these questions. I just want basic understanding for reasons this could or could not work.

Question 1

[… ] antibodies [… ] can last anywhere between a few weeks to a few months. The antibodies themselves though (which are proteins) definitely don't last forever (like all proteins). Eventually certain suppressor cells kill off plasma cells after the response has taken its course, and all that remains are a few memory cells, which are reactivated again should the pathogen invade the body in the future.


Question 2 and 3


  • Here is an article from Johns Hopkins University.

  • Here is a paper by Arturo Casadevall and Liise-anne Pirofski mentioned in the above article and published on the Journal of Clinical Investigation on March 13, 2020.

Fun fact: I was about to say in the question that I tried to research but didn't find anything. But I decided to make a last attempt, and actually found the answers…

Even Mild COVID-19 Cases Can Deliver Antibody Protection For Life, Study Finds

Radoslav Zilinsky via Getty Images

Even a mild case of COVID-19 can create antibodies that could give you lifetime protection from the virus, according to a new study.

“Months after recovering from mild cases of COVID-19, people still have immune cells in their body pumping out antibodies against the virus that causes COVID-19, according to a study from researchers at Washington University School of Medicine in St. Louis,” StudyFinds reports. “Such cells could persist for a lifetime, churning out antibodies all the while.”

The study’s findings were published on Monday in the journal, Nature. The researchers said there is evidence that immunity triggered by COVID-19 infection will be “extraordinarily long-lasting.”

“Last fall, there were reports that antibodies wane quickly after infection with the virus that causes COVID-19, and mainstream media interpreted that to mean that immunity was not long-lived,” said senior author Ali Ellebedy, an associate professor of pathology and immunology. “But that’s a misinterpretation of the data. It’s normal for antibody levels to go down after acute infection, but they don’t go down to zero they plateau. Here, we found antibody-producing cells in people 11 months after first symptoms. These cells will live and produce antibodies for the rest of people’s lives. That’s strong evidence for long-lasting immunity.”

Ellebedy said that after COVID-19 clears up, specialized immune cells called long-lived plasma cells move to the bone marrow, where they then manufacture low levels of COVID antibodies to help prevent reinfection with the virus.

Said the study: “Antibodies are proteins that can recognize and help to inactivate viral particles and are a key immune defense. After a new infection, short-lived cells called plasmablasts are an early source of antibodies. But these cells recede soon after a virus is cleared from the body, and other, longer-lasting cells make antibodies: memory B cells patrol the blood for reinfection, while bone marrow plasma cells (BMPCs) hide away within bones, trickling out antibodies for decades.”

“As expected, antibody levels in the blood of the COVID-19 participants dropped quickly in the first few months after infection and then mostly leveled off, with some antibodies detectable even 11 months after infection,” Washington University said. “Further, 15 of the 19 bone marrow samples from people who had had COVID-19 contained antibody-producing cells specifically targeting the virus that causes COVID-19. Such cells could still be found four months later in the five people who came back to provide a second bone-marrow sample. None of the 11 people who had never had COVID-19 had such antibody-producing cells in their bone marrow.”

“People with mild cases of COVID-19 clear the virus from their bodies two to three weeks after infection, so there would be no virus driving an active immune response seven or 11 months after infection,” Ellebedy said. “These cells are not dividing. They are quiescent, just sitting in the bone marrow and secreting antibodies. They have been doing that ever since the infection resolved, and they will continue doing that indefinitely.”

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Why Is There Such Reluctance to Discuss Natural Immunity?

If you’re among those of us who aren’t tribally invested in Covid politics but would like good information about when life will resume as normal, chances are you’re interested in herd immunity. You’re likely not interested in having to rely on the Internet Archive for good information on herd immunity. Alas, it’s become a go-to place for retrieving, as it were, previously published information on herd immunity that became inconvenient post-vaccine and then virtually Memory-Holed.

Over the past 15 months, the litany of Experts’ True Facts and Science regarding various aspects of SARS-CoV-2 has changed more often than the starting lineup of a bad minor league ball club. Covid-19 is spread by droplets, especially from asymptomatic people, until one day it was airborne all along and people who weren’t sick in all likelihood weren’t even sick. Stay at home, you’re safer indoors, even stay away from parks and beaches well, actually, outdoors is the place to be. Masks don’t work against viruses and are actually unhealthy to wear if you’re not sick, then suddenly they did work and without one you might as well be shooting people. Everyone knows and PolitiFact verified that the virus couldn’t have been created in the prominent infectious disease lab doing gain-of-function research on coronaviruses in bats coincidentally at Covid Ground Zero until, one day, PolitiFact had to retract the entire “Pants on Fire!” article. And so forth.

Unfortunately, information about herd immunity has also not been immune to this kind of meddling. Until recent months, people readily understood that active immunity came about either by natural immunity or vaccine-induced immunity. Natural immunity comes from battling and defeating an actual infection, then having your immune system primed for the rest of your life to fight it off if it ever shows up again. This immunity is achieved at a sometimes very high personal price.

Vaccine-induced immunity is to prime your immune system with a weaker, non-threatening form of the invading infection, so that it’s ready to fight off the real thing should you ever encounter it, and without your having first to risk severe illness or death.

Those interested in herd immunity in itself likely don’t have a moral or political preference for one form of immunity to the exclusion of the other. Immunity is immunity, regardless of whether a particular person has it naturally or by a vaccine. All immunity contributes to herd immunity.

Others, however, are much less circumspect. They seem to have forgotten the ultimate goal of the public campaign for people to receive vaccination against Covid-19. It’s not to be vaccinated it’s to have immunity. People with natural immunity — i.e., people whose immune systems have faced Covid-19 and won — don’t need a vaccine.

They do, however, need to be considered in any good-faith discussion of herd immunity. There are two prongs to herd immunity, as we used to all know, and those with natural immunity are the prong that’s being ignored. It’s not just mere oversight, however. Fostering such ignorance can lead to several bad outcomes:

  • People with natural immunity could be kept from employment, education, travel, normal commerce, and who knows what other things if they don’t submit to a vaccine they don’t need in order to fulfill a head count that confuses a means with the end
  • The nation could already be at herd immunity while governors and health bureaucrats continue to exert extreme emergency powers, harming people’s liberties and livelihoods
  • People already terrified of Covid — including especially those who’ve already had it — would continue to live in fear, avoiding human interaction and worrying beyond all reason
  • People could come to distrust even sound advice from experts about important matters, as they witness and grow to expect how what “the experts” counsel diverges from what they know to be wise counsel while it conforms to and amplifies the temporary needs of the political class

Those of us wanting good information certainly don’t want any of those outcomes. But others seem perfectly fine to risk them. They include not only elected officials, members of the media, political talking heads, self-important bureaucrats, and their wide-eyed acolytes harassing shoppers, but strangely also highly prominent health organizations.

For example, late last year Jeffrey Tucker showed that the World Health Organization (WHO) suddenly, and “for reasons unknown,” changed its definition of “herd immunity.” Using screenshots from a cached version on the Internet Archive, Tucker showed how the WHO altered its definition in such a way as to erase completely the role of natural immunity. Before, the WHO rightly said it “happens when a population is immune either through vaccination or immunity developed through previous infection.” The WHO’s change stated that it happens “if a threshold of vaccination is reached.” Not long after Tucker’s piece appeared, the WHO restored natural immunity to its definition.

The Food and Drug Administration (FDA), seemingly apropos of nothing, on May 19 issued a “safety communication” to warn that FDA-authorized SARS-CoV-2 antibody tests “should not be used to evaluate immunity or protection from COVID-19 at any time.” The FDA’s concern appears to be that taking an antibody test too soon after receiving a vaccination may fail to show vaccine-induced antibodies, but why preclude its use for “identifying people with an adaptive immune response to SARS-CoV-2 from a recent or prior infection?” Especially after stating outright that “Antibody tests can play an important role in identifying individuals who may have been exposed to the SARS-CoV-2 virus and may have developed an adaptive immune response.”

Then there is the National Institute of Allergy and Infectious Diseases director, Dr. Anthony Fauci, that ubiquitous font of fatuous guidance. He had told people that herd immunity would be at 60 to 70 percent immunity, and then he started publicly cinching those numbers up: 75 percent, 80 percent, 85 percent, even 90 percent (as if Covid-19 were as infectious as measles). He is quoted in the New York Times admitting to doing so deliberately to affect people’s behavior:

“When polls said only about half of all Americans would take a vaccine, I was saying herd immunity would take 70 to 75 percent,” Dr. Fauci said. “Then, when newer surveys said 60 percent or more would take it, I thought, ‘I can nudge this up a bit,’ so I went to 80, 85.“

Now — or better put, as of this writing — Fauci has taken to arguing herd immunity is a “mystical elusive number,” a distracting “endgame,” and therefore not worth considering. Only vaccinations are worth counting. As he put it recently, “We don’t want to get too hung up on reaching this endgame of herd immunity because every day that you put 2 million to 3 million vaccinations into people [it] makes society be more and more protected.”

While composing an article about natural immunity and herd immunity for my home state of North Carolina, I happened to notice that the Mayo Clinic had removed a compelling factoid about natural immunity. It’s something I had quoted in an earlier discussion of the matter and wanted to revisit it.

Here’s what the Mayo Clinic once wanted people to know in its page on “Herd Immunity and COVID-19” with respect to natural immunity: “[T]hose who survived the 1918 flu (influenza) pandemic were later immune to infection with the H1N1 flu, a subtype of influenza A.” The Mayo Clinic pointed out that H1N1 was during the 2009-10 flu season, which would be 92 years later. That finding attested to just how powerful and long-lived natural immunity could be.

As can be seen from the Internet Archive, however, sometime after April 14 the Mayo Clinic removed that compelling historical aside:

The Mayo Clinic also reoriented its page to feature vaccination over “the natural infection method” (method?) and added a section on “the outlook for achieving herd immunity in the U.S.” This new section stated that “it’s not clear if or when the U.S. will achieve herd immunity” but encouraged people nonetheless that “the FDA-authorized COVID-19 vaccines are highly effective at protecting against severe illness requiring hospitalization and death … allowing people to better be able to live with the virus.”

Why, from people who know better, is there so much interest in downplaying or erasing natural immunity?

Is it because it’s hard to quantify how many people have natural immunity? Is it out of a mix of good intentions and worry, that discussing natural immunity would somehow discourage (“nudge,” in Fauci’s term) people from getting vaccines who otherwise would? Is it simple oversight, being so focused on vaccinations that they just plain forgot about natural immunity? Or is something else at work?

Whatever the reason, it’s keeping Americans in the dark about how many people have active immunity from Covid-19. It’s keeping people needlessly fearful and suspicious of each other. It’s empowering executive overreach. Worst of all, it’s tempting people to consider government and business restrictions on the unvaccinated, regardless of their actual immunity.

Antibody Classes

Antibodies can be divided into five classes&mdashIgM, IgG, IgA, IgD, IgE&mdashbased on their physiochemical, structural, and immunological properties. IgGs, which make up about 80 percent of all antibodies, have heavy chains that consist of one variable domain and three identical constant domains. IgA and IgD also have three constant domains per heavy chain, whereas IgM and IgE each have four constant domains per heavy chain. The variable domain determines binding specificity and the constant domain of the heavy chain determines the immunological mechanism of action of the corresponding antibody class. It is possible for two antibodies to have the same binding specificities but be in different classes and, therefore, to be involved in different functions.

After an adaptive defense is produced against a pathogen, typically plasma cells first secrete IgM into the blood. BCRs on naïve B cells are of the IgM class and occasionally IgD class. IgM molecules make up approximately ten percent of all antibodies. Prior to antibody secretion, plasma cells assemble IgM molecules into pentamers (five individual antibodies) linked by a joining (J) chain, as shown in Figure (PageIndex<1>). The pentamer arrangement means that these macromolecules can bind ten identical antigens. However, IgM molecules released early in the adaptive immune response do not bind to antigens as stably as IgGs, which are one of the possible types of antibodies secreted in large quantities upon re-exposure to the same pathogen. Figure (PageIndex<2>) summarizes the properties of immunoglobulins and illustrates their basic structures.

Figure (PageIndex<2>): Immunoglobulins have different functions, but all are composed of light and heavy chains that form a Y-shaped structure.

IgAs populate the saliva, tears, breast milk, and mucus secretions of the gastrointestinal, respiratory, and genitourinary tracts. Collectively, these bodily fluids coat and protect the extensive mucosa (4000 square feet in humans). The total number of IgA molecules in these bodily secretions is greater than the number of IgG molecules in the blood serum. A small amount of IgA is also secreted into the serum in monomeric form. Conversely, some IgM is secreted into bodily fluids of the mucosa. Similar to IgM, IgA molecules are secreted as polymeric structures linked with a J chain. However, IgAs are secreted mostly as dimeric molecules, not pentamers.

IgE is present in the serum in small quantities and is best characterized in its role as an allergy mediator. IgD is also present in small quantities. Similar to IgM, BCRs of the IgD class are found on the surface of naïve B cells. This class supports antigen recognition and maturation of B cells to plasma cells.


Jenelle Eli, Director of International Communications for American Red Cross told Reuters via email that people who are eligible to give plasma donations can do so if they have been vaccinated against COVID-19. “This applies to all COVID-19 vaccines currently authorized in the U.S. including J&J, Moderna and Pfizer.”

Eli pointed to an exception: “if an individual received a non-FDA approved COVID-19 vaccine as part of a clinical trial [and] the vaccine was a live attenuated COVID-19 vaccine [meaning it contains a weakened live virus [ here , here ] or the individual does not know what kind of vaccine it was, they must wait two weeks.”

Eli also clarified that all blood donors must feel healthy when they donate, so people who are experiencing the side effects from the vaccine will have to wait until their symptoms get better to donate.

Plasma Donation

Doctors can use plasma to treat different kinds of serious health problems.

Some of the elements in plasma, including the antibodies and chemicals that help your blood to clot, can help in medical emergencies like burns and trauma.

Other things that plasma donation is good for include:

  • Developing treatments. The antibodies and proteins can also be used to develop treatments for rare diseases, including some immune system problems.
  • Cancer. Adults and children with different kinds of cancer -- including leukemia -- sometimes need plasma transfusions.
  • Transplant surgery. Some people who get liver or bone marrow transplants need plasma.
  • Hemophilia. In this rare disorder, a person’s blood doesn’t have enough clotting factors, so donated plasma can help.

Had COVID? You’ll probably make antibodies for a lifetime

A bone-marrow plasma cell (artificially coloured). Such cells, which produce antibodies, linger for months in the bodies of people who have recovered from COVID-19. Credit: Dr Gopal Murti/Science Photo Library

Many people who have been infected with SARS-CoV-2 will probably make antibodies against the virus for most of their lives. So suggest researchers who have identified long-lived antibody-producing cells in the bone marrow of people who have recovered from COVID-19 1 .

The study provides evidence that immunity triggered by SARS-CoV-2 infection will be extraordinarily long-lasting. Adding to the good news, “the implications are that vaccines will have the same durable effect”, says Menno van Zelm, an immunologist at Monash University in Melbourne, Australia.

Antibodies — proteins that can recognize and help to inactivate viral particles — are a key immune defence. After a new infection, short-lived cells called plasmablasts are an early source of antibodies.

But these cells recede soon after a virus is cleared from the body, and other, longer-lasting cells make antibodies: memory B cells patrol the blood for reinfection, while bone marrow plasma cells (BMPCs) hide away in bones, trickling out antibodies for decades.

“A plasma cell is our life history, in terms of the pathogens we’ve been exposed to,” says Ali Ellebedy, a B-cell immunologist at Washington University in St. Louis, Missouri, who led the study, published in Nature on 24 May.

Has COVID peaked? Maybe, but it’s too soon to be sure

Researchers presumed that SARS-CoV-2 infection would trigger the development of BMPCs — nearly all viral infections do — but there have been signs that severe COVID-19 might disrupt the cells’ formation 2 . Some early COVID-19 immunity studies also stoked worries, when they found that antibody levels plunged not long after recovery 3 .

Ellebedy’s team tracked antibody production in 77 people who had recovered from mostly mild cases of COVID-19. As expected, SARS-CoV-2 antibodies plummeted in the four months after infection. But this decline slowed, and up to 11 months after infection, the researchers could still detect antibodies that recognized the SARS-CoV-2 spike protein.

To identify the source of the antibodies, Ellebedy’s team collected memory B cells and bone marrow from a subset of participants. Seven months after developing symptoms, most of these participants still had memory B cells that recognized SARS-CoV-2. In 15 of the 18 bone-marrow samples, the scientists found ultra-low but detectable populations of BMPCs whose formation had been triggered by the individuals’ coronavirus infections 7–8 months before. Levels of these cells were stable in all five people who gave another bone-marrow sample several months later.

“This is a very important observation,” given claims of dwindling SARS-CoV-2 antibodies, says Rafi Ahmed, an immunologist at Emory University in Atlanta, Georgia, whose team co-discovered the cells in the late 1990s. What’s not clear is what antibody levels will look like in the long term and whether they offer any protection, Ahmed adds. “We’re early in the game. We’re not looking at five years, ten years after infection.”

Ellebedy’s team has observed early signs that Pfizer’s mRNA vaccine should trigger the production of the same cells 4 . But the persistence of antibody production, whether elicited by vaccination or by infection, does not ensure long-lasting immunity to COVID-19. The ability of some emerging SARS-CoV-2 variants to blunt the protective effects of antibodies means that additional immunizations may be needed to restore levels, says Ellebedy. “My presumption is, we will need a booster.”

Updates & Corrections

Correction 27 May 2021: An earlier version of this article gave the wrong number of bone-marrow samples. This has now been corrected.

Can antibody tests tell you if a COVID-19 vaccine worked?

If you’ve been vaccinated against COVID-19, you may be thinking about getting an antibody test to see if the vaccine “worked.” Or, if you donate blood at MD Anderson Blood Bank or elsewhere, you may get back your antibody test results after you donate blood.

Antibody testing identifies individuals who may have developed an immune response after infection with SARS-CoV-2, the coronavirus responsible for COVID-19 disease.

But can these antibody test results really tell you if the COVID-19 vaccine was effective? Here’s why you shouldn’t be surprised to get a negative result even if you’ve received a COVID-19 vaccine.

What your COVID-19 antibody test results mean

A positive result suggests that you had COVID-19 at some time in the recent past, even if you didn’t show any symptoms. You’ll need to continue to practice caution in the community and protect yourself and others from potential exposure to COVID-19 by following safety precautions, including masking, social distancing and handwashing.

A negative result suggests that you haven’t had a recent COVID-19 infection. However, it doesn’t prove that there has been no prior or current infection. Antibodies could be present at levels below the test’s threshold for detection. It takes one to three weeks after an infection for antibodies to be detectable. In the months after an infection, antibody levels may decrease below the detectable level.

For either result, if you’re experiencing cough, fever, difficulty breathing, sore throat, loss of taste/smell, chills/muscle pain and/or gastrointestinal symptoms, self-isolate and contact your doctor.

Antibody testing isn’t conclusive for predicting COVID-19 immunity

While serology testing for COVID-19 is increasing in the United States, experts still have a lot of questions about what can be determined from the results.

Despite several versions of the antibody tests on the market from multiple manufacturers, current evidence suggests antibody testing alone isn’t conclusive for diagnosing a person with current or prior COVID-19 or for predicting a patient’s sustained immunity. It remains particularly unclear how long antibodies exist in a person’s body following a COVID-19 infection and how many antibodies are needed to neutralize SARS-CoV-2 following another exposure.

Can antibody tests determine if the COVID-19 vaccine was effective?

The Centers for Disease Control and Prevention (CDC) discourages antibody testing for assessing immunity after getting the vaccine.

A vaccinated person is very likely to get a negative result from a serology test, even if the vaccine was successful and protective. That’s because different serology tests detect antibodies to different parts of the virus.

Some tests detect antibodies to the spike protein of the virus, which are produced in response to viral infection or the vaccine. Others detect antibodies to a different part of the virus called the nucleocapsid protein, which are produced in response to infection, but not by the current vaccines.

MD Anderson’s Blood Bank uses an antibody test designed to detect antibodies to the nucleocapsid protein, which means donors who have received the COVID-19 vaccine will likely receive a negative antibody test result.

Learning plasma’s effectiveness, on a large scale

Much more recently than 1918, convalescent plasma therapy was tried for treatment of H1N1, Ebola and the Zika virus. Emma Meagher, vice dean and chief clinical officer at Penn’s Perelman School of Medicine, said that in those outbreaks, the data on plasma’s effectiveness as a treatment is not very extensive. For example, there was a lack of potential donors researchers could draw from with Zika, which was mostly contained to South America, largely Brazil.

At Penn now, a two-part study is underway looking into the effectiveness of the treatment with COVID-19.

The first aspect of the study is a donor protocol, in which researchers are identifying people who previously had COVID-19 infections and are now at least 28 days’ symptom-free. Once someone is identified, the team invites that individual to Penn’s apheresis, or transfusion medicine suite so it can obtain one unit of plasma. The level of antibodies in that unit of plasma is then measured.

The second part of the study is the clinical trial protocol, in which patients currently in the hospital for COVID-19 treatment receive plasma. Meagher said that for this part researchers are interested in two patient populations: those who are critically ill in the intensive care unit and need mechanical ventilation, and patients who are less sick and not in the ICU but are still hospitalized.

This part of the study is based on enrolling 50 patients. Those who are critically ill will receive a unit of plasma on top of their regular COVID treatment. The results would be compared to a plasma study being conducted at the National Institutes of Health that has a placebo component.

Patients who are less severely ill will be part of a randomized placebo control trial. Half the patients will receive the regular standard of care and the other half will receive the plasma along with their normal treatment.

“The end points we are looking for in the clinical trial is to see if the antibody titers in the recipients — the patients who receive the infusion — rise and remain elevated,” said Meagher, who is helping to lead the study. “And we are also interested in seeing if those increases in antibodies correlate with an improved clinical outcome. Do they get off the ventilator earlier? Do they have a shorter hospital stay? Is their pulmonary function improved over a shorter period of time in comparison to a control group?”

By mid- to late May, Meagher said, researchers will have a good sense of the plasma treatment response and will be able to confidentially share their results so far with other centers.

The clinical trial component was recently greenlighted by the FDA through the request of an emergency investigational new drug application. She said her team planned to start treating patients this week.

One unique feature of the clinical trial also will include antibody testing.

Scott Hensley, a viral immunologist and microbiology professor at Penn, has helped develop a test to determine whether someone has ever been infected by the coronavirus. Hundreds of health care workers at the Hospital of the University of Pennsylvania have given blood so far, with the hope of determining if they have some level of immunity to the virus.

“We want to test patients serially so we can actually determine what the time course of antibody development is, which is really important to us in predicting the time course of recovery and allowing us to estimate how quickly people can return to health,” Meagher said. “We’re going for precision here. We want to try to see if the antibody levels themselves correlate with improved outcome.”

The aim is to roll the treatment out first at HUP and Penn Presbyterian, Meagher said, since they are two of the Penn health system’s most affected hospitals in the region right now.

Three hundred donors will be sought over the next four weeks — a high goal, she said, though finding eager and eligible donors is the least of their problems.

“We get people are in a spirit of giving right now and everyone wants to help in some way, and so we have lots of us fielding calls every day of patients who have recovered, friends of friends, college kids reaching out to say that they would like to do something,” Meagher said.

People who think they might be eligible can reach out through the Penn Medicine website.

In speaking with other health systems that have started the treatment, such as Mount Sinai and NYU Langone Medical Center, Meagher said, they are hearing anecdotally from physicians who have given plasma to COVID-19 patients that there have been no adverse effects and no toxicities so far.

“But they are very cautious to say it’s working, and I think that’s partly because it’s not being done in a sort of rigid experimental model where there isn’t a controlled experiment,” Meagher said.

Why can a type O person donate blood to all other blood types but can only receive type O blood?

It's because type O blood lacks any antigens, and therefore a person with type O blood has A, B, and Rh antibodies, assuming they are type O-negative.


Blood cells have antigens on their surface that serve as markers or flags, and plasma contains antibodies that detect and reject blood cells with foreign antigens.

Antigens (A, B, and Rh)
There are several antigens present on the surface of blood cells that serve as "markers" or "flags." These include A, B and Rh antigens. For instance, if a person has A and B antigens present in their blood, their blood type is deemed "type AB," while blood with only A antigens will be deemed "type A." If a person's blood contains certain Rh factors, they are deemed "Rh positive" or just "positive." Therefore, a person with type "AB-positive blood" has the A, B, and Rh antigens. If person's blood contains neither A nor B antigens, they are deemed "type O," so a person with "O positive" blood has neither A nor B antigens, but would still have Rh antigens.

For every antigen in a person's blood, they lack the antibody for that specific antigen. For instance, a person who is "A-positive" will have B antibodies, but neither A nor Rh antibodies. Similarly, a person with blood type "AB-negative" will have neither A nor B antibodies, but will still have Rh antibodies.

Why do you have to donate blood to someone with the same blood type?
You don't have to donate blood to someone with the same blood type. All that is necessary for a donor to be a "match" is that their blood must not trigger any of the antibodies in the recipient's blood. For instance, a person with blood type AB-negative cannot donate to a person with blood type B-negative, because the recipient's blood will have A antibodies present, and will reject the blood since it contains A antigens. Here's a table describing who can receive blood whom:

Since someone who is "AB-positive" will lack all blood-type related antibodies in their blood, they can receive blood from anyone. For this reason, people with blood type "AB-positive" are deemed "universal recipients." In stark contrast, since someone who is "O-negative" will have no antigens in their blood, they can donate to anyone. For this reason, people with blood type "O-negative" are called "universal donors." However, a person with type "O-negative" blood with have all the other antigens in their bloodstream and can only receive type O-negative blood.