Which technique is based on the principle of antigen-antibody interaction?

ELISA

Correct, ELISA (Enzyme-Linked Immunosorbent Assay) is indeed based on the principle of antigen-antibody interaction.

Infection by a pathogen can be detected in two main ways:

1. Detection of Antigens: During an infection, pathogens (such as bacteria, viruses, or other microorganisms) produce specific molecules known as antigens. Antigens are usually proteins or glycoproteins found on the surface of the pathogen. These antigens are recognized by the host's immune system as foreign invaders, triggering an immune response.

In diagnostic tests like ELISA, specific antibodies that recognize and bind to these antigens are used. The assay involves immobilizing the antibodies on a solid surface (e.g., a microplate) to create a capture antibody. When a sample suspected of containing the pathogen (e.g., a blood sample) is added to the plate, any present antigens from the pathogen will bind to the immobilized capture antibodies. Then, another set of antibodies, called detection antibodies, which are linked to an enzyme, are added. These detection antibodies bind to different sites on the antigen, forming an antigen-antibody complex. Finally, a substrate is added that the linked enzyme converts into a detectable signal, such as a colored product. The signal indicates the presence of the pathogen's antigen in the sample.

1. Detection of Antibodies: When the body encounters a pathogen, the immune system produces specific proteins called antibodies in response to the infection. These antibodies are tailored to recognize and bind to the antigens present on the pathogen's surface. The presence of specific antibodies against a particular pathogen indicates a current or past infection by that pathogen.

Various serological tests, including ELISA, can detect the presence of antibodies against a pathogen in a patient's blood sample. In these tests, antigens from the pathogen are used as probes, and the patient's serum (blood component containing antibodies) is exposed to these antigens. If antibodies against the pathogen are present in the serum, they will bind to the antigens, and this binding can be detected using labeled secondary antibodies specific for human antibodies. The presence of these antibodies indicates exposure to the pathogen, either in the past or currently.

Both antigen and antibody detection methods are valuable in diagnosing infections, monitoring immune responses, and guiding appropriate treatment decisions.