ELISA (Enzyme-linked immunosorbent assay)

Enzyme-linked immunosorbent assay - or ELISA for short - is a laboratory technique or assay where antibodies or antigens in people’s samples are immobilized in a surface, and then detected by an antibody with an enzyme attached that causes a change of color.

So it’s useful to diagnose infections, such as HIV, hepatitis B, or malaria; and autoimmune diseases, like Graves’ disease, systemic lupus erythematosus, or rheumatoid arthritis, where the body reacts to its own proteins as if they were foreign antigens.

Immune response

We are constantly surrounded by harmful microorganisms called pathogens, that could cause a lot of damage if it wasn’t for the immune system.

These pathogens have unique parts called antigens.

Now, B lymphocytes are cells of the immune system that can detect those antigens and trigger an immune response by secreting lots and lots of antibodies.

These antibodies, also called immunoglobulins or Ig, are Y-shaped proteins that have two regions: the constant fragment region, also called Fc, that determines the antibody class - IgD, IgM, IgG, IgA, or IgE; and two fragment-antigen binding, or Fab regions, that recognize and bind to the antigen in order to inactivate it, preventing the pathogen from reaching its target cell and causing damage.

Principle of ELISA

Now, the basic idea with ELISA is to use that specific link between the antigen and antibody to help diagnose infections or autoimmune diseases.

More specifically, ELISA can be used to look for either the pathogen’s antigens or the antibodies our body secretes against them.

More than that, it also makes them visible or measurable by using an antibody against the suspected antigen or antibody.

But this antibody comes with a +1: it has an enzyme attached, which can modify a substrate called a chromogen.