An antigen is a molecule capable of inducing an immune response , i.e., to produce an antibody in the host organism.

In turn, antigens are targeted by antibodies.

Each antibody is specifically produced by the immune system to match an antigen.

This allows a precise identification/matching of the antigen and the initiation of a tailored response.

The antibody is said to "match" the antigen in the sense that it can bind to it due to an adaptation performed to a region of the antibody;

Because of this, many different antibodies are produced, each with specificity to bind a different antigen while sharing the same basic structure.

In most cases, an adapted antibody can only react to and bind one specific antigen; in some instances, however, antibodies may cross-react to and bind more than one antigen

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An epitope, also known as an antigenic determinant, is the specific part of an antigen molecule to which an antibody or a T-cell receptor binds. Epitopes are typically short sequences of amino acids on the surface of proteins, although they can also be portions of polysaccharides, lipids, or nucleic acids. There are two main types of epitopes:

1. **Linear (or Sequential) Epitopes**: These consist of a continuous sequence of amino acids in a protein. The primary structure of the protein (its amino acid sequence) is sufficient for the immune system to recognize these epitopes.

2. **Conformational (or Discontinuous) Epitopes**: These are composed of amino acids that are not in a continuous sequence but are brought together in the three-dimensional structure of the protein. The protein’s secondary and tertiary structures are crucial for the formation of these epitopes.

Epitopes play a crucial role in the immune response, as they are the specific sites on antigens that are recognized by antibodies, B cells, and T cells. Understanding epitopes is important in vaccine design, diagnostics, and therapeutic antibody development.

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