A major class of non-specific antibacterial proteins induced in the hemolymph of the pupae of silk moths, particularly Hyalophora cecropia, as part of the immune response. (See also Cecropins.) Originally designated as immune protein P5, attacins are a heterogeneous family consisting of four basic (A,B,C,D) and two acidic (E,F) forms with similar amino acid sequences and mol wts of 20,000-23,000 daltons. Initial identification and isolation of immune response proteins: I. Faye et al., Infect. Immun. 12, 1426 (1975). Purification and preliminary characterization: A. E. Pye, H. G. Boman, ibid. 17, 408 (1977). Amino acid sequences and antibacterial activity: D. Hultmark et al., EMBO J. 2, 571 (1983). Structural study on attacin F: . Engstrm et al., ibid. 3, 2065 (1984). Mode of action: P. Engstrm et al., ibid. 3347.