Final answer:
The mechanism for inactivation of autoreactive B cells associated with downregulation of surface IgM in the periphery is anergy, which aids in preventing autoimmunity. Option D.
Step-by-step explanation:
The mechanism for inactivation of autoreactive B cells in the periphery that is associated with downregulation of surface IgM is known as anergy. Anergy is a state of non-responsiveness to antigen stimulation due to the absence of necessary co-stimulatory signals needed for activation.
In this situation, the B cell encounters a self-antigen but is not provided with essential signals from helper T cells (Th2), leading to anergy.
This lack of response prevents the self-reactive B cells from becoming fully activated and thereby helps to protect against autoimmune diseases by inhibiting unintended immune reactions against the body's own tissues.
Peripheral tolerance is another key mechanism that plays a part in preventing autoimmunity. It involves the regulation of not only T cells but also B cells. When maturing in the bone marrow, B cells that recognize self-antigens are either deleted through apoptosis or made nonfunctional through clonal anergy.
Upon entering the periphery, if self-reactive B cells fail to receive signals from Th2 cells, they may enter an anergic state, undergo apoptosis, or their receptors are edited to no longer react with self. So option D.