A heterophile antigen is best described as:

A heterophile antigen refers to an antigen that is found in different species which are not necessarily related. This concept stems from the ability of certain antigens to provoke an immune response across different types of organisms, often due to structural similarities in their molecules.

Option B is correct because heterophile antigens can be present in unrelated animals, which means that the immune system of one species might recognize antigens from another, leading to cross-reactivity. This phenomenon is commonly seen in certain serological tests, such as those for infectious mononucleosis, where antibodies can react to antigens from various sources.

The other options do not accurately describe heterophile antigens. The idea that the antigen exists in humans only is incorrect, as heterophile antigens are by definition not species-specific. The notion of an antigen being associated with a specific pathogen misrepresents the broader nature of heterophile antigens—they can be non-specific and not limited to pathogens that cause disease. Finally, the concept of a protein being unique to certain diseases contrasts with what heterophile antigens represent; they are not unique but rather shared by multiple organisms. Thus, heterophile antigens are best characterized by their presence in unrelated animals, making option B the most fitting choice.