Antitoxin component of a type II toxin-antitoxin (TA) system. Antitoxin that binds cognate toxin SpoIISA and neutralizes its toxic activity; unlike most antitoxins it does not seem to be highly labile upon expression in E.coli. The isolated protein is unfolded; X-ray data suggests the inactive complex forms a heterotetramer of SpoIISA(2)-SpoIISB(2), which inactivates the toxic activity of SpoIISA. Expressed during exponential growth and sporulation. A member of the spoIISA-spoIISB operon, it also has its own promoter. Decreases sporulation efficiency by 4 orders of magnitude. A double spoIISA-spoIISB disruption sporulates normally, suggesting its only role is to neutralize SpoIISA. Belongs to the SpoIISB antitoxin family.