**5. Specific receptor binding**

Receptors are attachment sites for ligand molecules i.e. polypeptide hormones and neurotransmitters. In case of antigen–antibody complex formation, antigen expression on cell surface is considered a definite receptor site for antibody binding. Antigen fragments are present on the upper surface and within the cells or sometimes released into body fluids. Localization of definite receptor-binding radiotracers depends upon multiple factors such as blood clearance, affinity of the tracer & receptor and blood flow of the tumor tissue. Receptors are of various types with specificity of basic compound including peptides, steroid hormones, and antibodies.
