The capsule of Bacillus anthracis behaves as a thymus-independent type 2 antigen.

Bacillus anthracis elaborates a homopolymeric capsule composed of gamma-D-glutamic acid residues. Mice were immunized with formalin-fixed encapsulated B. anthracis bacilli, and the serum antibody response to a gamma-D-glutamyl capsular epitope was measured. Antiglutamyl antibodies were elicited in athymic BALB/c Nu/Nu, BALB/c Nu/+, and CBA/J mice but not in CBA/N xid mice. These response patterns define the capsule of B. anthracis as a thymus-independent type 2 antigen.