Final answer:
The Fc region of an antibody molecule is responsible for its immune effector functions, engaging in processes such as opsonization and ADCC, by binding with complement factors and Fc receptors on immune effector cells.
Step-by-step explanation:
The immune effector function of an antibody (Ab) is primarily mediated by the Fc region of the molecule. The Fc region is part of the constant region of the antibody and comprises the trunk of the Y-shaped molecule as well as the lower portion of each arm. This region is critical for interactions with complement factors and phagocytic cells during antibody-mediated opsonization. Another key function involving the Fc region is antibody-dependent cell-mediated cytotoxicity (ADCC), which is crucial for eliminating pathogens that cannot be phagocytosed. In ADCC, the Fc receptors on effector cells such as natural killer (NK) cells, macrophages, and eosinophils bind to the Fc region of antibodies that are attached to a pathogen, facilitating the killing of the pathogen by the immune system.
Meanwhile, the Fab region of the antibody is involved in the specificity of antigen binding. Each antibody has two Fab regions which contain the antigen-binding sites. This region determines the specific epitope that the antibody will bind to, but it is the Fc region that endows the antibody with its effector functions, allowing it to interact with other cells in the immune system and trigger a response against the pathogen.