What is the main function of CD55 or decay accelerating factor (DAF)?

The main function of CD55, also known as decay accelerating factor (DAF), is to inhibit complement activation. CD55 plays a crucial role in regulating the complement cascade, which is part of the innate immune response. By binding to C3 and C5 convertases, CD55 prevents their formation, thus slowing down the activation of the complement pathway. This action helps to protect host cells from potential damage caused by complement-mediated lysis and inflammation, which can occur during an immune response.

Inhibition of complement activation is essential for maintaining homeostasis and preventing autoimmune reactions where the body's own cells might be attacked. Without the regulatory effects of CD55, there could be uncontrolled complement activation leading to tissue damage and various autoimmune disorders.

The other potential functions listed, such as promoting apoptosis, aiding T-cell activation, and facilitating B-cell differentiation, do not directly relate to the primary role of CD55 as a complement regulatory protein. Therefore, the emphasis on its role in inhibiting complement activation correctly identifies its key function in immune regulation.