A primitive T cell-independent mechanism of intestinal mucosal IgA responses to commensal bacteria

The immunoglobulin A (IgA) is produced to defend mucosal surfaces from envi ronmental organisms, but host defenses against the very heavy load of intes tinal commensal microorganisms are poorly understood. The IgA against intes tinal commensal bacterial antigens was analyzed; it was not simply "natural antibody" but was specifically induced and responded to antigenic changes within an established gut flora. In contrast to IgA responses against exoto xins, a significant proportion of this specific anti-commensal IgA inductio n was through a pathway that was independent of T cell help and of follicul ar lymphoid tissue organization, which may reflect an evolutionarily primit ive form of specific immune defense.