An antigen is a substance either foreign or internal that triggers an immune response. This response is triggered by a histocompatibility molecule within each cell that can trigger an immune response.
C'mon, it's obviously A. Antigens are like the supervillains of the body - they're here to cause trouble, not help with digestion or muscle movement. I bet the person who wrote this question was feeling a little 'antsy' that day.
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