Fc is which fragment of the antibody molecule?

The Fc fragment of the antibody molecule is referred to as the crystallizable region. This term arises from the fact that this portion can form stable crystals, allowing for easier study of its structure and function. The Fc region plays a crucial role in the immune response; it is responsible for mediating interactions with various cell surface receptors and proteins, including those on immune cells, such as macrophages and natural killer (NK) cells. The Fc fragment is important for functions such as opsonization, complement activation, and antibody-dependent cellular cytotoxicity (ADCC).

In contrast, the variable region of the antibody (which is found in the Fab fragment) is responsible for antigen binding and is composed of the light and heavy chains that contribute to the specificity of the antibody. The term "amino-terminal" does not specifically relate to the Fc fragment but rather describes the end of a polypeptide chain that has a free amine group. Understanding the structural components of antibodies, including the Fc fragment's role, is fundamental in immunology and the design of therapeutic antibodies.