agent vs stressor

agent and stressor both are nouns.

Word	Noun	Adjective	Verb	Adverb
agent	Yes	No	No	No
stressor	Yes	No	No	No

As nouns, stressor is a hyponym of agent; that is, stressor is a word with a more specific, narrower meaning than agent:

agent (noun)	stressor (noun)
the semantic role of the animate entity that instigates or causes the happening denoted by the verb in the clause	any agent that causes stress to an organism
an active and efficient cause; capable of producing a certain effect
a businessman who buys or sells for another in exchange for a commission
a representative who acts on behalf of other persons or organizations
any agent or representative of a federal agency or bureau
a substance that exerts some force or effect