During an allergic reaction, the binding of antigens to IgE antibodiess initiates a response, in which chemicals cause the dilation of blood vessels and a host of other physiological changes. Such chemicals are

Options

A. interferons
B. hormones
C. histamine
D. acetylamine

Explanation

**Step-by-Step Solution:**

1. **Understanding the Context**: During an allergic reaction, the immune system responds to what it mistakenly identifies as harmful substances (antigens). These antigens bind to specific antibodies in the body.

2. **Identifying the Antibody**: The primary antibody involved in allergic reactions is Immunoglobulin E (IgE). This antibody is produced by B lymphocytes and is crucial in mediating allergic responses.

3. **Mechanism of Action**: When antigens bind to IgE antibodies, it triggers immune cells, particularly mast cells and basophils, to release various chemicals.

4. **Identifying the Chemicals**: The key chemical released during this process is histamine. Histamine is derived from the amino acid histidine and is released from mast cells during an allergic reaction.

5. **Effects of Histamine**: Histamine causes several physiological changes, including the dilation of blood vessels (vasodilation) and increased permeability of blood vessel walls. This leads to common allergic symptoms such as swelling, redness, itching, sneezing, and runny eyes/nose.

6. **Conclusion**: The chemical responsible for the physiological changes during an allergic reaction, particularly the dilation of blood vessels, is histamine.

**Final Answer**: The chemicals that cause the dilation of blood vessels during an allergic reaction are histamines.

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