Foreign materials, bacteria, viruses, fungi, and parasites are all considered:

The correct answer is that foreign materials, bacteria, viruses, fungi, and parasites are considered antigens. Antigens are substances recognized by the immune system as foreign, triggering an immune response. They can come from various sources, including pathogens like bacteria and viruses, or be components of larger structures like fungi and parasites. The immune system identifies these antigens to mount a defensive reaction, which may involve the production of antibodies.

In contrast, the other options do not align with the definition of antigens. Cancers generally refer to abnormal cell growth and are not classified as foreign materials. Antibodies are proteins produced by the immune system in response to antigens, thus serving as the body's defense mechanism rather than being considered foreign substances themselves. Allergies involve an exaggerated immune response to harmless substances known as allergens, which do not encompass the broader range of foreign materials listed in the question.