antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that recognizes the epitope is called a paratope. Although epitopes are usually thought to be derived from non-self proteins, sequences derived from the host that can be recognized are also classified as epitopes.
The epitopes of protein antigens are divided into two categories, conformational epitopes and linear epitopes, based on their structure and interaction with the paratope. A conformational epitope is composed of discontinuous sections of the antigen's amino acid sequence. These epitopes interact with the paratope based on the 3-D surface features and shape or tertiary structure of the antigen. Most epitopes are conformational.
By contrast, linear epitopes interact with the paratope based on their primary structure. A linear epitope is formed by a continuous sequence of amino acids from the antigen.
The result of doctors injecting keyhole limpet hemolymph (KLH) is a poweful immune response. The immune system can be stimulated to attack a certain cancer by attaching markers to KLH. A real plus by using KLH is that it is nontoxic.
As you know before using a vaccine for cancer treatment clinical trials have to performed. A treatment for bladder cancer is now approved in Europe and Asia.
You can read more in Popular Science article written by Sam Roudman on the December 2011 issue.