White blood cells patrol the bloodstream constantly, searching for potential threats. When _____ encounter a bacterium or virus, these immune...

Let's begin by understanding the meaning of this sentence. We'll use our understanding of pause points and segment the sentence as shown - understanding and assimilating the meaning of each segment bit by bit!

Sentence Structure

Sentence 1:

• White blood cells
                patrol the bloodstream constantly,
                searching for potential threats.

Sentence 2:

• When [?] encounter a bacterium or virus,
• these immune defenders
                can quickly mobilize a coordinated attack,
                releasing chemical signals
                               that amplify the body's defensive response.

Sentence 3:

• Dr. Marcus Webb
                studies how this cellular communication network operates.

Understanding the Meaning

The passage starts by telling us about white blood cells:

• 'White blood cells patrol the bloodstream constantly, searching for potential threats.'
  • So we have these cells that are constantly on guard, looking for dangers like bacteria or viruses.

Now we reach the second sentence, which starts with the blank:

• 'When _____ encounter a bacterium or virus...'

This is where we have the blank. Let's look at the choices:

• it / those / they / she

These are all pronouns - words that refer back to something mentioned earlier. To see what works here, we need to figure out what noun this pronoun is referring to!

Looking back at what we've read:

• The most recently mentioned noun is 'white blood cells'
  • This is plural - we're talking about multiple cells

Now let's check what the sentence structure tells us:

• The phrase is 'When [pronoun] encounter a bacterium or virus'
  • 'encounter' is the plural form of the verb
  • If it were singular, we'd see 'encounters'
  • So the pronoun needs to be plural too

What do we notice?

• We need a plural pronoun that refers back to 'white blood cells'
• 'They' is the natural choice - it's the standard way to refer back to a plural noun we already mentioned
• 'Those' is also plural, but it's used to point to or distinguish specific items - it would sound awkward here because we're simply continuing to talk about the same cells
• 'It' and 'she' are both singular, so they don't match

So we need they.

Now let's read the rest of the sentence to see the complete picture:

• 'these immune defenders can quickly mobilize a coordinated attack, releasing chemical signals that amplify the body's defensive response.'
  • Notice it says 'these immune defenders' - this is another way of referring to the white blood cells
  • When they encounter a threat, they launch an attack and send out signals to strengthen the body's defense

The final sentence tells us:

• Dr. Marcus Webb studies how this cellular communication network operates.
  • A researcher is investigating this communication system

GRAMMAR CONCEPT APPLIED

Matching Pronouns to the Nouns They Refer Back To

When you use a pronoun (a word like "it," "they," "she," or "those") to refer back to a noun you mentioned earlier, that pronoun needs to match the original noun in number - singular or plural. This matching relationship is called pronoun-antecedent agreement in grammar terms.

The Pattern:

• Singular noun → Singular pronoun
  • "The cell detects a threat. It sends a signal."
  • "cell" (singular) → "it" (singular)

• Plural noun → Plural pronoun
  • "The cells detect a threat. They send signals."
  • "cells" (plural) → "they" (plural)

In our question:

• First sentence introduces: "White blood cells" (plural)
• Second sentence needs to refer back: "When they encounter..." (plural pronoun)
• The pronoun "they" correctly matches "white blood cells" in number

Why "they" rather than "those"?

• "They" is a personal pronoun - the standard way to continue referring to something already mentioned
• "Those" is a demonstrative pronoun - used to point to or distinguish specific items
• When simply continuing to talk about the same noun, "they" is the natural choice