The Big Bang Theory: 10 Things You Completely Missed From Season One
4. Is Sheldon Allergic To Cats Or Not?
In the third episode, The Fuzzy Boots Corollary, Leonard feels depressed after he is rejected by Dr. Leslie Winkle and Penny. To accommodate his increasing loneliness, Leonard strongly considers getting a cat. Sheldon immediately shuts down this notion, alerting his flatmate that his asthma is triggered by cat fur.
Even though Sheldon's feline allergy is referenced a number of times in later episodes, it seems to be thrown out the window in Season 4. In the episode, The Jazzy Substitution, Sheldon has broken up with his girlfriend, Amy Fowler, encouraging him to purchase 25 cats to take his mind off her. It's funny (and super cute) to see the unemotional Sheldon childishly play with his clowder but it also contradicts Sheldon's claim that he's severely allergic to cats. Since he emphasised being around one kitten could trigger an asthma attack, playing with two dozen of his furry friends should cause his lungs to explode.
Nevertheless, none of the characters point out Sheldon's initial disdain for cats in the past and his allergy isn't mentioned for the rest of the show's run.