What are pathogens?

Pathogens are disease-causing agents. They can be bacteria, viruses, fungi, protists, or parasitic worms.
They can infect other organisms and cause various diseases.
Choice A is wrong because phagocytic leukocytes are a type of white blood cell that can engulf and destroy pathogens.
They are part of the immune system and help fight infections.
Choice B is wrong because antibodies are proteins that can bind to specific antigens on the surface of pathogens and mark them for destruction by other immune cells.
They are also part of the immune system and help fight infections.
Choice D is wrong because agents that alter DNA base sequences are not necessarily pathogens.
They can be mutagens, such as chemicals or radiation, that can cause changes in the DNA of cells.
Some mutagens can also be carcinogens, which can cause cancer.

Paracrine secretions are chemical signals that affect only nearby cells.

They are released by the sending cell and diffuse through the extracellular space to the target cell, which has a specific receptor for the signal.
Paracrine signaling is a form of local signaling that occurs in multicellular organisms.
Choice A is wrong because autocrine secretions are chemical signals that act on the same cell that produced them.
Autocrine signaling is also a form of local signaling, but it involves cell signaling itself.
Choice B is wrong because endocrine secretions are chemical signals that are secreted into the blood and act at long distances.
Endocrine signaling is a form of long-range signaling that involves ductless glands that secrete hormones.
Choice C is wrong because exocrine secretions are non-hormonal substances that are secreted into ducts to the external environment.
Exocrine signaling is not a form of cell-cell communication, but rather a way of releasing substances such as sweat, saliva, or digestive enzymes.