The blood contains five major types of WBCs, or __. Their functin is t fight infections. SOme are __, engulifing and digesting microorganisms, as well as debris fromt eh bod's own dead cells. Other __, called __,d evelop into specialized B and T cells that mount immune responses against foreing substances. unlike __, __ are also found outside the circulatory ststem, patrolling both interstitial fluid and the lymphatic system.