The hemagglutinin (HA) component of the progenitor toxin protects the structural integrity of botulinum neurotoxin; may increase internalization of the neurotoxin into the bloodstream of the host (PubMed:9421908). The HA component is involved in binding to the upper small intestine through interactions with glycolipids and glycoproteins containing sialic acid moieties (Probable). Botulinum toxins are produced as progenitor toxins of large molecular sizes of 12S (M toxin) and 16S (L toxin). M toxin consists of a non-toxic, non-hemagglutinin component (NTNHA) and the neurotoxin (Probable). L toxin consists of the M toxin and the 3 subcomponents of hemagglutinin (HA) (PubMed:7802661). HA is composed of subcomponents of 70, 33, and 17 kDa (PubMed:7802661). The 70 kDa subcomponent undergoes proteolytic processing and is split into HA-55 (also called HA-53 and HA3b) and HA-22-23 (also called HA3a) (PubMed:7802661). Preliminary crystallization studies show a tri-lobed HA-70 where each HA-70 contacts one HA-17 molecule (Probable). The stoichiometry of the whole complex has been modeled as one BoNT/C, one NTNHA, three HA-70, six HA-33 and three HA-17 (By similarity). This protein can also be encoded on a prophage.