Slytherin Defense Club

Blood Purity

The Sorting Hat claims that blood purity is a factor in selecting Slytherins.
This suggests that it bases its decisions more upon the views of its members than on any accurate test of a student's lineage.

Muggle-born Slytherins exist, but are very rare, as noted insultingly by Scabior the Snatcher. There have also been definite examples of half-bloods sorted into the house, including Tom Marvolo Riddle, Dolores Umbridge, and Severus Snape. Harry Potter was nearly sorted into Slytherin, but because he asked for it, he ended up in Gryffindor instead.

Also, pure-bloods are not automatically placed in Slytherin. There have been various examples of pure-bloods sorted into other houses, including the: Weasley family, Sirius Black, Neville Longbottom, and James Potter all being sorted into Gryffindor house, Ernie Macmillan being sorted into Hufflepuff, and Morag MacDougal and Stephen Cornfoot being sorted into Ravenclaw.
Contrary to the wishes of the other founders of Hogwarts School of Witchcraft and Wizardry, Salazar Slytherin wanted a stricter policy of admission to the school —one limited strictly to pure-blood students. The controversy this created apparently led to him leaving the school and the Chamber of Secrets behind. A millennium later, the house's students are still considered affiliated with Salazar's reputed views, and open expression of pro-blood purity sentiment does seem to be more common in Slytherin than in other Houses. Also, no one can really blame him for not letting Muggleborns into Hogwarts, when Muggles were actively trying to kill all wizards, regardless of intent at the time.

This disagreement over blood purity, which caused Salazar Slytherin's friendship with Godric Gryffindor to break, is the origin of the feud between Slytherin and Gryffindor, the two most competitive of the houses. Slytherin and Gryffindor are generally rivals in Quidditch and for the House Cup, which the former had won seven years in a row prior to Harry Potter's arrival at Hogwarts in 1991. It is also rare for Slytherin and Gryffindor students to be friendly with one another, and common for them to share animosity. This was particularly the case during the First and Second Wizarding Wars. During the second war, no Slytherin students joined an association known as Dumbledore's Army; however, this could be because only those Hermione Granger, Ron Weasley, and Harry Potter trusted were invited, as the three shared a deep mistrust of any Slytherins. Some Slytherins declined to defend their school in the Battle of Hogwarts, (although they were forced out of the school when Voldemort attacked) with some joining their families in becoming Death Eaters. However, there are exceptions, such as Slytherin Severus Snape and Gryffindor Lily Evans, who were best friends until Snape's use of the slur "Mudblood" and his aspirations to be a Death Eater caused Lily to sever their ties.