Short answer. Quite a lot. Section 13 of Rule 6 lets a third-party defendant raise defenses, counterclaims, or cross-claims in the answer, including defenses the third-party plaintiff has against the original plaintiff's claim, and even a counterclaim against the original plaintiff in proper cases.
What the law says
A third (fourth, etc.)-party defendant may allege in his or her answer his or her defenses, counterclaims or cross-claims, including such defenses that the third (fourth, etc.)-party plaintiff may have against the original plaintiff's claim.
Rule 6, Section 13 — Answer to third (fourth, etc.). Read the full provision →
The third-party defendant's own defenses
Section 13 confirms that a third-party defendant — someone brought into an existing case through a third (or fourth, etc.) party complaint — is not limited to a narrow response. They may allege in their answer their own defenses, counterclaims, or cross-claims, just as any other defending party could, addressing the claim the third-party plaintiff has made against them specifically.
Borrowing the third-party plaintiff's defenses against the original claim
More distinctively, Section 13 allows the third-party defendant to raise such defenses that the third-party plaintiff may have against the original plaintiff's claim. This reflects the derivative nature of third-party liability in many cases: if the third-party plaintiff's exposure depends on their liability to the original plaintiff, the third-party defendant has an interest in — and is permitted to raise — the same defenses the third-party plaintiff could have used against that original claim.
A counterclaim reaching back to the original plaintiff
Section 13 goes further still, allowing that in proper cases, the third-party defendant may assert a counterclaim against the original plaintiff directly, in respect of the original plaintiff's claim against the third-party plaintiff. This creates a chain of potential claims running from the original plaintiff through the original defendant (as third-party plaintiff) to the third-party defendant and back again, all resolvable within the single case.
Why this matters for how the case unfolds
Because a third-party defendant can raise their own defenses, borrow the third-party plaintiff's defenses against the original claim, and even counterclaim directly against the original plaintiff, the whole chain of related claims can generally be resolved in one proceeding rather than several. This spares the parties from having to litigate a separate follow-on case just to sort out liability once the original dispute is settled, and lets the court weigh the connected claims together with a fuller picture of who is ultimately responsible for what. None of this expands what the third-party defendant may be held liable for beyond the scope of the third-party complaint itself; the answer is where these defenses and claims must be raised, and a third-party defendant who fails to plead an available defense there risks losing the chance to raise it later in the same proceeding.