Question

a person who has a weakened immune system may not be able to fight against infection because
pathogens may enter the body and cause illness.
phagocytes may enter the body and cause illness.
normal cells crowd out unhealthy cells.
antibodies production is increased.

Explanation:

The immune system's main role is to defend against pathogens. A weakened immune system fails to do so effectively. Phagocytes are part of the immune defense (they engulf pathogens), so them entering to cause illness is incorrect. Normal cells crowding out unhealthy cells is not related to immune weakness. Increased antibody production would help, not hinder. Pathogens entering and causing illness due to a weak immune system makes sense.

Answer:

pathogens may enter the body and cause illness.