Which of the following is an example of a pathogen?

A virus that causes colds is classified as a pathogen because it is an organism or agent that is capable of causing disease in a host. Pathogens include various types such as viruses, bacteria, fungi, and parasites. The common cold is primarily caused by viruses such as rhinoviruses, making them directly responsible for the illness and its transmission.

In contrast, the other choices represent different biological entities or functions that do not cause disease. For example, a protein that provides immunity is part of the immune response that helps protect against pathogens rather than being a pathogen itself. Antibiotics are medications designed to treat bacterial infections, but they are not pathogenic organisms. White blood cells are crucial components of the immune system that work to combat infections, again not falling under the category of pathogens themselves. Therefore, the correct answer points to the nature of pathogens and their role in disease causation.