Which phrase most accurately describes antigens?

• A. Substances that activate complement only
• B. Usually components of invading microbes or foreign substances
• C. Regions of antibodies that bind antigens
• D. Self molecules only

Answer: (B)

Antigens are substances that the immune system recognizes as non-self and that can trigger an immune response. They are usually parts of invading microbes or other foreign materials the body encounters, which the B and T cells identify via their receptors and mount a response against. This description fits best because it captures the typical source and purpose of an antigen—foreign components that the immune system targets.

The other ideas don’t define what an antigen is: some substances can activate complement, but that isn’t the defining feature of an antigen; the regions that bind antigens are parts of antibodies (the binding sites), not the antigens themselves; and while self molecules can occasionally become targets in autoimmune conditions, antigens are generally foreign rather than self.