<prompt> The exact pa of the antigen that reacts with the immune system is called asCloneEpitopeIdiotopeEffector  <response>: Option 1, 2 Epitope: Epitope or antigenic determinant is the smallest unit of antigenicity: 1. It is defined as a small area present on the antigen that is capable of sensitizing T- and B-cells and reacting with specific site of T-cell receptor or an antibody. 2. The specific site of an antibody that reacts with the corresponding epitope of an antigen is called as paratope. Option 3 Idiotypes: The unique amino acid sequence present in paratope region in VH and VL regions of one member of a species acts as antigenic determinant to other members of the same species: o Such antigenic determinants are called as idiotopes. Option 4 Functions of Immunoglobulins: 1. Antigen binding by Fab region 2. Effector functions by Fc region- Fixation of complement: Antibody coating the target cell binds to complement through its Fc receptor which leads to complement mediated target cell lysis. Binding to various cell types: Phagocytic cells, lymphocytes, platelets, mast cells, NK cell, eosinophils and basophils bear Fc receptors FcR that bind to Fc region of immunoglobulins. 