Please answer a question about the following article about Northwestern University:

Northwestern was founded in 1851 by John Evans, for whom the City of Evanston is named, and eight other lawyers, businessmen and Methodist leaders. Its founding purpose was to serve the Northwest Territory, an area that today includes the states of Ohio, Indiana, Illinois, Michigan, Wisconsin and parts of Minnesota. Instruction began in 1855; women were admitted in 1869. Today, the main campus is a 240-acre (97 ha) parcel in Evanston, along the shores of Lake Michigan just 12 miles north of downtown Chicago. The university's law, medical, and professional schools are located on a 25-acre (10 ha) campus in Chicago's Streeterville neighborhood. In 2008, the university opened a campus in Education City, Doha, Qatar with programs in journalism and communication.

In what year did Northwestern University began teaching?
1855