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Capsules can help bacteria avoid host defenses by limiting phagocytosis, complement deposition, and neutrophil killing. They also promote attachment to tissues and surfaces, supporting biofilm formation, and can protect against desiccation and bacteriophages. Capsule antigens vary by type and are relevant to immune responses, vaccine design, and serotyping. Several clinically important pathogens are encapsulated; patients with reduced splenic function are at increased risk of fulminant infection from these organisms. Laboratory approaches include the Quellung reaction, which uses anticapsular antibody to produce visible capsule swelling, and negative staining to reveal a halo. Capsule genes may be transferred between bacteria, while their loss or mutation can reduce virulence.