A 20-year-old man who has multiple sexual partners and does not use barrier precautions has had a non-tender ulcer on his penis for the past week. On physical examination, the 0.6-cm lesion has a firm, erythematous base and sharply demarcated borders. The lesion is scraped, and the microscopic darkfield examination is positive for motile spirochetes. Which of the following inflammatory processes is most likely to accompany this infection?