A ______is a liquid substance which contains dead or weakened antigens.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a term that describes a liquid substance containing dead or weakened antigens.

2. **Identifying Key Terms**: The key terms in the question are "liquid substance," "dead," "weakened," and "antigens." These terms are commonly associated with immunology and vaccines.

3. **Recalling Relevant Knowledge**: In biology, particularly in the context of immunology, a substance that contains dead or weakened forms of pathogens (like bacteria or viruses) is known to stimulate an immune response without causing the disease.

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