Final answer:
Macrophages are immune cells known for phagocytizing pathogens, serving as antigen-presenting cells, and activating T cells, but they do not secrete substances onto pathogens that are too large to ingest.
Step-by-step explanation:
The function that is NOT a characteristic of macrophages is C. Macrophages secrete substances onto pathogens that are too large to ingest. Macrophages are well-known for their abilities to phagocytize pathogens, which means they engulf and digest foreign particles. They also serve an essential role in the immune response as antigen-presenting cells (APCs). After engulfing a pathogen, macrophages present parts of the pathogen on their surface via the Major Histocompatibility Complex (MHC) Class II molecules. This process activates T cells, specifically helper T cells, which then can help in the further activation of other immune cells.
While macrophages do kill ingested pathogens with chemicals within their phagolysosomes and activate T cells by presenting antigens, the secretion of substances onto large pathogens is not traditionally ascribed to macrophages.