checkpoint

blockade of PD-1 lets T-cells whack
some cancers that had learned to skate on by
disregulating aut-immune attack
malignant tumors--sometimes--melt and die
redundant safeguards keep the dogs in check
to ward self-tissues from such civil war
instead of passing through one bottleneck
with PD-1, there's CTLA-4
but when such interventions go awry
T-leukocytes proliferate in haste
whole host self-tissues shrivel up and die
immune attack lays the whole patient waste
immuno-onco's no mere passing fad
but when it misses, things go really bad