The function of DEFA4 (defensin alpha 4, Ensembl gene identifier ENSG00000164821) is as follows. Host-defense peptide that has antimicrobial activity against Gram-negative bacteria, and to a lesser extent also against Gram- positive bacteria and fungi (PubMed:15317502, PubMed:15616305, PubMed:2500436, PubMed:2501794, PubMed:30658057). Exhibits antimicrobial activity against Gram-negative E.coli and E.aerogenes and Gram-positive S.faecalis, S.aureus and B.cereus and the yeast C.albicans (in vitro) (PubMed:15317502, PubMed:15616305, PubMed:17088326, PubMed:2500436, PubMed:2501794, PubMed:30658057). Interacts with pathogenic surface proteins and toxins, such as HIV-1 surface protein gp120 and B.anthracis anthrax lethal factor lef (PubMed:15620707, PubMed:30658057). Protects blood cells against infection with HIV-1 (in vitro) (PubMed:15620707). Inhibits enzymatic activity of B.anthracis lef/anthrax lethal factor (in vitro) (PubMed:30658057). Inhibits corticotropin (ACTH)-stimulated corticosterone production (in vitro) (PubMed:2843187). {ECO:0000269|PubMed:15317502, ECO:0000269|PubMed:15616305, ECO:0000269|PubMed:15620707, ECO:0000269|PubMed:17088326, ECO:0000269|PubMed:2500436, ECO:0000269|PubMed:2501794, ECO:0000269|PubMed:2843187, ECO:0000269|PubMed:30658057}.