What are the specific reactive sites on an antigen called?

The specific reactive sites on an antigen are called determinants, often referred to as epitopes. These are the specific regions or structures on the antigen that are recognized by the immune system, particularly by antibodies or T-cell receptors. Each determinant has a unique shape or configuration that allows it to interact specifically with the immune components that recognize it.

Determinants can be composed of various types of molecular structures, including linear sequences of amino acids or specific three-dimensional configurations formed by the folding of the protein. The recognition of these sites is critical for the immune response, as it leads to the activation of B cells and T cells, ultimately resulting in the production of antibodies or the activation of cytotoxic T cells.

In contrast, the other choices refer to different concepts. Sites of mutation pertain to alterations in the genetic sequence of DNA or RNA, affecting the antigen structure but not directly describing their reactive sites. Cell receptors are molecules on the surface of cells that bind to specific ligands, such as antigens, but do not refer to the antigen's reactive sites themselves. Linkages may pertain to how molecules are connected or associated but do not specifically describe the functional sites on the antigen that interact with the immune system. Thus, "determinants" correctly