Basophils and eosinophils are cells related to the neutrophil (see above). When activated by a pathogen encounter, basophils releasing histamine are important in defense against parasites, and play a role in allergic reactions (such as asthma). When they are activated, eosinophils secrete a range of highly toxic proteins and free radicals that kill bacteria and parasites. The same chemicals also cause tissue damage during allergic reactions. Activation and toxin release by eosinophils is therefore tightly regulated to prevent any inappropriate tissue destruction.