In schistosome immunity, which stage is primarily attacked by the immune system?

• A. Eggs
• B. Worms
• C. Cercariae
• D. Miracidia

Answer: (A)

In schistosome infections, the immune response is driven most strongly by the eggs. When adult worms lay eggs that get trapped in tissues like the liver, intestine, or bladder, the eggs release antigens that provoke a robust Th2-type response. This leads to recruitment of eosinophils, macrophages, and the formation of granulomas around the eggs. The granulomatous reaction is the main source of pathology, including fibrosis, and it reflects the body’s targeted attack on the egg stage. The other stages—the free-swimming cercariae and miracidia, and the adult worms—tend to evade or bypass intense immune attack for various reasons (short-lived presence, intracellular phases, or immune-masking strategies), so they don’t drive the same level of immune reaction as eggs do.