A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the immune system. It does not need another sentence for context. It gives a broad overview of what the immune system does. Why other lines are not chosen? Paragraph 2 starts with "When" which is a conjunction, so it cannot be independent. Paragraph 3 starts with "However" which is also a conjunction. Paragraph 4 starts with "Thus" which is a transitional word, so it cannot stand alone.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the main ideas. It shows the importance of studying the immune system. It gives a conclusion to the discussion.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the idea of the immune system. It explains how the immune system works when it is functioning properly. This shows a logical order.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it shows the opposite idea. It talks about what happens when the immune system does not work well. This is a cause and effect relationship.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains why studying the immune system is important. It shows the need for understanding the immune system to help with diseases. This is a logical order.