Conformational epitope

In immunology, a conformational epitope is a sequence of sub-units (usually amino acids) composing an antigen that come in direct contact with a receptor of the immune system.

An antigen is any substance that the immune system can recognize as foreign.

Proteins are composed of repeating nitrogen-containing subunits called amino acids that in nature do not exist as straight chains but as folded whorls with complex loops.

So, whenever a receptor interacts with an undigested antigen, the surface amino acids that come in contact may not be continuous with each other if the protein is unwound.

In contrast, if the antigen is digested, small segments called peptides are formed, which bind with major histocompatibility complex molecules, and then later with T cell receptors through amino acids that are continuous in a line.