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What are antibodies?

a coronavirus being attacked by antibodies
Antibodies are Y-shaped proteins that bind to the body's foreign invaders and signal the immune system to get to work.
(Image: © Shutterstock)

Antibodies are specialized, Y-shaped proteins that bind like a lock-and-key to the body's foreign invaders — whether they are viruses, bacteria, fungi or parasites. They are the "search" battalion of the immune system's search-and-destroy system, tasked with finding an enemy and marking it for destruction.

"They're released from the cell and they go out and hunt," said Dr. Warner Greene, the director of the Center for HIV Cure Research at the Gladstone Institutes in San Francisco.


Related: Diagram of the human immune system (infographic)

What do antibodies look like?

The two arms at the top of the antibody's Y shape bind to what's known as the antigen. The antigen can be a molecule, or a molecular fragment — often some part of a virus or bacteria. (For instance, the new coronavirus SARS-CoV-2 has unique "spikes" on its outer coat, and some antibodies bind to and recognize these spike proteins.) 

The bottom of the Y, or the stalk, binds to several other immune-system compounds that can help kill the antigen or mobilize the immune system in other ways. One set of these, for instance, triggers the complement cascade, Greene told Live Science.

"Complement is actually the executioner," that punches holes in the target cell, such as the membrane of a virus, Greene said. 

Antibodies, which are also called immunoglobulins (Ig), all have the same basic Y-shape, but there are five variations on this theme — called IgG, IgM, IgA, IgD and IgE, said Jason Cyster, a professor of microbiology and immunology at the University of California, San Francisco. 

Each variation looks slightly different and plays slightly different roles in the immune system. For instance, immunoglobulin G, or IgG, is just one Y, whereas IgM looks a bit like the 10-armed Hindu goddess Durga, with five Ys stacked together, and each prong can bind one antigen.

IgG and IgM are the antibodies that circulate in the bloodstream and go into solid organs, Cyster said. IgA is "squirted out of the body," in mucus or secretions, Cyster told Live Science. IgE is the antibody that typically triggers allergic responses, such as to pollen or peanuts, according to the American Academy of Allergy, Asthma & Immunology. IgD has historically been enigmatic, but one of its roles is to help activate the cells that make antibodies.

Related: Hay fever & seasonal allergies: Symptoms, causes & treatment

Antibodies are Y-shaped proteins. The two arms at the top of the Y bind to the intruder molecule. The bottom of the Y, or the stalk, binds to several other immune-system compounds that can help kill the intruder or signal the immune system to take care of it in other ways. (Image credit: Shutterstock)

Where do antibodies form?

To understand antibodies, you first need to know about B-cells, which are a type of white blood cell that forms in the bone marrow. There are about a trillion B-cells in the body, and each one has a unique IgM antibody that sits on the B-cell surface and each binds, to one antigen, said Simon Goodman, the Science and Technology Program Manager for The Antibody Society, a nonprofit organization that represents those involved in antibody research and development.

This staggering level of variation allows the body to recognize almost any substance that could enter. Here’s how it achieves that diversity: In each B-cell, the genes that code for the antibody's binding site are shuffled like playing cards in a deck.

"The amount of rearrangement that can occur is enormous," Cyster told Live Science. 

Related: 11 surprising facts about the immune system

These B-cells then patrol the body, often lingering longer in areas like the lymph nodes or the tonsils, Cyster said. Most of the time, these B-cells don't bind anything. But if, by a one- in-a-million chance, a B-cell does bind some foreign substance, "that triggers the B-cell to say 'Hey we need to get activated,'" Cyster said.

The B-cell grows in size and starts to divide in what's called "clonal expansion," Cyster said.

"It's an identical copy of the parent, just like the mother," Cyster said. After a week or so, there may be hundreds of thousands to a million of these copies.

Eventually, these clonally expanded B-cells differentiate into plasma cells, which are antibody factories.

"They secrete 10,000 antibodies per cell per second. They can do that for weeks or years if you're lucky," Cyster said. 

But not all B-cells divide the same amount. 

"If you consider the B-cell to be a lock, and you consider all of these different things to be floating around to be different keys, then some of the keys will fit better, some will fit worse, and some won't fit at all," Goodman told Live Science. "And depending on how well the key fits into the lock on the surface of a particular B-cell, that cell will be triggered to divide more." Then, the more prolific B-cells produce more plasma cells and churn out more of a specific type of antibody.

The body doesn't just produce one type of antibody either; it produces a messy, chaotic zoo of them. Each locks onto different parts of an invader. 

Drawings of a B-cell, T-cell, antibodies and a macrophage. (Image credit: Shutterstock)

And antibodies don't all do the same thing once they've bound to a target. Some will nip infection in the bud by directly neutralizing a threat, preventing a pathogen from entering a cell. Others tag invaders, so that the immune system's killer cells (which aren’t antibodies) can remove it, Greene said. Still others may wrap viruses or bacteria in a gooey coating. And other antibodies might tell Pac-Man-like immune cells called macrophages to come gobble up the invader. (That strategy can sometimes backfire with viruses, which may co-opt this response to invade new cells, Cyster added.)

The first type of antibody to form after you are exposed to a virus is IgM, which emerges within 7 to 10 days after exposure, Greene said. IgM can bind to an invader, but each "Y" in this 10-armed protein does so fairly weakly. But, just as five weak people working together can tackle a large, strong adversary, IgM's five Y's (10 arms) working together can bind tightly to an antigen, he added.

At about 10 to 14 days, the body begins making IgG, which is the immune system's "major workhorse," Greene said. IgG can cross the placenta in a pregnant woman, giving a newborn passive protection against disease until their own immune system can ramp up, Greene added.

Normally, the immune system is stunningly good at recognizing the enemy and ignoring, or tolerating, our own cells. Sometimes, however, this process goes awry. That’s when T-cells (another type of white blood cells) come in. The body uses these T-cells to cross-check targets — only if both a B-cell and a T-cell recognize something as a foreign invader will an immune response be triggered, Goodman said. The body is supposed to remove B-cells that make so-called auto-antibodies, which react to the body's own cells. But when that doesn't happen, the body may mark its own cells for destruction and then relentlessly eliminate them. Autoimmune diseases such as lupus, rheumatoid arthritis, or type 1 diabetes can result, Goodman said. There are more than 100 autoimmune disorders, according to the American Autoimmune Related Diseases Association.

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How do antibody tests work? 

Antibody tests detect whether the body has produced detectable quantities of antibodies to a certain molecule, and can therefore reveal whether someone has been infected by a specific virus or bacteria in the past. Usually, these tests are detecting IgM or IgG, Live Science previously reported.

For instance, SARS-CoV-2 antibody tests typically detect either part or all of the coronavirus' spike protein and can reveal whether someone has had COVID-19 in the past. Because the body takes time to ramp up its production of antibodies, people usually only test positive about two weeks after they were first exposed to the pathogen, Live Science previously reported.

Related: Can antibody tests tell if you're immune to COVID-19?

There are two common types of antibody tests — lateral flow assays and enzyme-linked immunosorbent assay (ELISA) tests. Both involve fixing an antigen to a surface and then detecting whether an antibody binds to that antigen. Usually, a chemical reaction, such as fluorescence or a color-change, is triggered when the antibody binds to the antigen. Lateral flow assays are similar to pee-on-a-stick pregnancy tests; rather than pee, for antibody tests, blood or serum is washed over the flat surface, which is usually paper. ELISA tests work on a similar principle, only the tests are conducted in microplates and require a lab technician, and the results may not read out instantly, Charlotte Sværke Jørgensen, who studies Virus and Microbiological Special Diagnosis Serology at the Statens Serum Institut in Copenhagen, previously told Live Science in an email.

A good antibody test is one that produces few false positives and few false negatives, Live Science previously reported. To ensure that happens, scientists need to "calibrate" their test, for instance, by making sure that samples known to not have the antigen do not falsely produce a positive test. For instance, with SARs-CoV-2, that would mean testing blood samples from before the pandemic started and making sure no samples come up positive. They also need to take samples that definitely have the antibody in them, and make sure the antibody test does a good job of detecting those positives. 

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