The investigators also wish to identify and characterize the sub-population of high-threshold peanut-allergic individuals who have clinical reactivity to peanut allergen but only at higher doses and characterize their immunological phenotype in comparison with the low-threshold peanut allergic individuals enrolled in the adult PN OIT study (2012P002153/AADCRC MGH-004). Some of the The investigators will identify these high-threshold peanut allergic and sensitized, but non-allergic individuals by recruiting subjects who enrolled in 2012POO2153 but failed to meet all of the inclusion criteria - specifically, they failed to have objective symptoms during a food challenge with a cumulative total dose of 443 mg. The investigators will then clarify their status by conducting a double-blind placebo-controlled food challenge to peanut, using higher doses of peanut. The investigators will draw blood before and after this food challenge, and collect urine and stool at the time of the challenge for studies of immune mechanisms.