Final answer:
d. Antigens are foreign substances that can induce an immune response, resulting in the generation of antibodies. They are recognized by immune cells like macrophages, which process and present the antigens to other immune cells.
Step-by-step explanation:
Outside or foreign substances capable of causing disease are known as d. antigens. Antigens are typically macromolecules that the immune system identifies as nonself.
Upon detection, the immune response is triggered and may result in the production of specific proteins called antibodies which bind to the antigens and target pathogens for destruction by other parts of the immune system.
Not all antigens will incite an immune response; the body can develop tolerance to harmless antigens present within the environment, such as food proteins, pollen, or dust components.
An antigen is detected by macrophages, which are a type of immune cell capable of engulfing and processing these foreign substances to present them to other immune cells for identification and response.