Felisyl Immune System Support for Cats - Amino Acid L-Lysine - Produce Antibodies - Healthy Tissue, Respiratory Health, Vision - 60 Soft Chews Felisyl is a soft chew supplement that contains the amino acid L-Lysine for use in cats and kittens for conditions responsive to L-Lysine L-Lysine is an essential amino acid, which means that your pet's body needs it, but can't produce it, so this supplement is a perfect choice to provide that necessity L-Lysine HCL Immune System Support for Cats helps keep your cat healthy by supporting the immune system naturally so that it can function normally and produce antibodies