Antibodies are blood circulating glycoproteins termed immunoglobulins that are secreted by plasma cells (differentiated B lymphocytes) in response to immune stimulation. They constitute what is known as the humoral immune response, which is highly specific. Alternatively, B lymphocytes may differentiate into "memory cells" which display antibody molecules on their cell surface contributing to active immunity for future exposure to an antigen. There are five major classes of antibodies that differ upon their location in the body, structure, and function: IgA, IgD, IgM, IgG, IgE. During a typical bacterial or viral infection, the major classes that operate are IgM and IgG.
Antibodies recognize and bind tightly to small molecules (peptide or carbohydrate) termed antigens. Antigens may be free floating in the blood, or bound to the plasma membrane of a pathogen or presented at the surface by another immune cell. An antibody may directly neutralize a pathogen, mark a microbe for attack, or be used to bind to antigens presented by other cells for further immune stimulation.
Antibodies play a significant role in immune response. When a human body finds a new single pathogen, it is typically capable of destroying it immediately. However, when an infection is found, a stronger response is often required. The body works to create antibodies for that type of pathogen, which will then enable the full immune system to identify and destroy all copies of that pathogen. Once distributed through the body, these antibodies also help the immune system to prevent reinfection. If the same type of pathogen enters the same body again, the antibodies will quickly mark it as hostile, and it will be destroyed, typically before it is able to start another infection. For this reason, most pathogens can only successfully start an infection in each person one time. Only if it mutates into a significantly different form can it enter that body again soon after, without being promptly detected. This means that the body is said to be "immune" to such pathogens once successfully fighting off an infection. Vaccines attempt to imitate this process by either faking an infection, or causing a small infection, thus hopefully prompting the body to make antibodies for that pathogen.
- Antibody testing is showing that a lot of Americans already have had the virus and recovered from it.
- Antibody tests can reveal who is immune to a disease, but they can also determine how widely it has spread and how deadly it is. When will we reopen the country? Antibody testing may help officials decide, experts say