• Role of Decay-Accelerating Factor (CD55): CD55 (Decay-Accelerating Factor, DAF) is a 70-kDa glycosylphosphatidylinositol (GPI)-anchored cell surface regulatory glycoprotein that protects normal host autologous cells from accidental complement-mediated lysis.
• Mechanism: CD55 binds to membrane-bound C3 convertases of both the classical/lectin pathway ($C4b2a$) and alternative pathway ($C3bBb$), accelerating their rapid dissociation and preventing formation of the terminal membrane attack complex (MAC).
• Contrasting Complement Regulators:
• CD59 (Protectin / MAC-IP): Binds directly to C8 and C9, blocking polymerisation of the MAC pore.
• CD21 (CR2): Complement receptor 2 on B cells that binds C3d.
• CD23: Low-affinity receptor for IgE (Fc$\epsilon$RII).

Correct option: (1) CD55

• Titer Determination in Complement Fixation Test (CFT): In the standard diagnostic Complement Fixation Test, the antibody titer is formally defined as the highest dilution of patient serum at which no more than 50% of the sensitized sheep red blood cells (SRBCs) are lysed (i.e., at least $50\%$ complement fixation / inhibition of hemolysis occurs).
• Assay Principle: Antigen and serum antibody form immune complexes that consume (fix) added guinea pig complement. When hemolysin-sensitized SRBCs (indicator system) are added, unconsumed complement causes hemolysis. Hence, lack of hemolysis ($\lt 50\%$) indicates complement was bound by specific antibodies.

Correct option: (1) No more than 50% of the red cells are lysed