What happens when an antibody binds an antigen?

a. it causes pathogens to stick together.

b. it increases body temperature.

c. it produces cytotoxic t cells.

d. it starts the inflammatory response.

A. It causes pathogens to stick together.

Explanation:

Antigens are the antibody generating substances present on the surface of the pathogen such as bacteria, viruses and fungi.  When an antibody binds to an antigen (proteins on the surface of pathogens), it makes the pathogens to stick together. This limit the movement of the pathogens around the body and this allows phagocytes to engulf and then destroy them easily.

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nucleus

would be most reasonable answer tbh