Substances known as provoke a specific immune response so discriminating that only a single molecular fragment, called an actually interacts with the lymphocyte’s receptor.

Antigens, and epitopes.

Explanation:

Antigen is known as a substances which is capable of the causes an immune response in the body, and specifically activating white blood cells which are known as body's infection fighting cells. In general antigens are two types:

1) Foreign antigens or heteroantigens.

2) Auto-antigens or self antigens.

Epitopes are the part of an antigen which is recognized with the help of immune system, specifically by B cells, T cells, and antibodies. They are also called as antigenic determinants. The epitope is defined as the specific part of the antigen where an antibody binds.

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