What bacterial structure inhibits phagocytosis?

• A. Capsule
• B. Flagellum
• C. Pili
• D. Cell wall

Answer: (A)

Capsules act as anti-phagocytic virulence factors. The capsule forms a slimy, polysaccharide-rich outer layer around certain bacteria, which masks surface structures and reduces the binding of phagocytes as well as opsonizing antibodies and complement. This makes engulfment by macrophages and neutrophils less likely, allowing the bacteria to persist in tissues and cause disease. Encapsulated organisms such as Streptococcus pneumoniae, Klebsiella pneumoniae, and Haemophilus influenzae type b illustrate this mechanism. Other structures like flagella (motility), pili (attachment), or the cell wall (structural features) do not primarily inhibit phagocytosis, so they are less directly involved in avoiding uptake by immune cells.