In a serological test for syphilis, the addition of which substance would cause a glowing reaction if antibodies are present?

In a serological test for syphilis, the presence of antibodies indicative of infection can lead to a glowing reaction when certain substances are added. The correct answer involves both red blood cells (RBCs) and anti-human globulin.

The glowing reaction, or agglutination, occurs because anti-human globulin (also known as Coombs reagent) binds to human antibodies that are attached to the surface of RBCs. In the context of a syphilis test, if specific antibodies produced in response to T. pallidum (the causative agent of syphilis) are present in the serum, these antibodies will bind to the RBCs. Once anti-human globulin is introduced, it will attach to these bound antibodies, resulting in visible agglutination. This combination of RBCs and anti-human globulin enhances the detection of antibodies, thereby inducing a notable reaction.

The inclusion of only RBCs would not create a glowing reaction unless bound by these antibodies, and using anti-human globulin alone would not generate a response without the presence of target antibodies attached to RBCs. Therefore, both components are essential for the observed agglutination in this context, making the combination the correct choice.