The boys are sitting at the Cheesecake Factory, and Sheldon and Raj are "Tressling", a combination of playing the Tetris video game while simultaneously arm wrestling. Penny asks the guys to knock it off, then leaves them to participate in a brief birthday ritual at another customer's table. Raj is winning the Tetris portion, and Sheldon is winning the arm wrestling. Ultimately, they declare a tie and quit. When Penny returns, she asks if Leonard will be coming into the restaurant to claim a free cheesecake of his own to celebrate his own birthday in a few days. Sheldon points out that Leonard is lactose intolerant and can't have the cheesecake, or the cream cheese frosting on the carrot cake (Penny points out that he could just scrape the frosting off of his cake).Leonard wants to know how Penny knew his birthday was coming up, and Penny reminds him that she had done his horoscope.She was going to do the whole gang, except that Sheldon threw a fit. Sheldon defends his position that horoscopes have been scientifically denounced as hokum. Penny rolls her eyes and calls Sheldon "blah blah blah....a typical Taurus". Leonard tells Penny that he probably won't come in because he doesn't celebrate his birthday; his mother taught him to celebrate personal achievements, and being expelled through a birth canal wasn't one. Leonard admits that he has never had a birthday party.
The next day, as Leonard leaves to pick up Thai food, Penny goes to talk to Sheldon and the boys to plan a surprise party for Leonard's birthday. Sheldon wants to nip this in the bud, noting that Leonard said he didn't WANT a party. Penny is convinced that this is because Leonard doesn't know what he's missing. Sheldon thinks that NOT having the party would be merciful, based on the emotional scars of his own birthday party experiences. Penny blackmails Sheldon, threatening to open his mint comic books and draw in them in ink if he doesn't participate. Sheldon agrees to throw Leonard a birthday party.
Howard and Raj shop for presents and they meet back at Penny's apartment. Sheldon berates them for getting the secret knock wrong. Sheldon hasn't secured a gift, making a big rant about why...until Howard has Penny inform Sheldon that gift giving is a non-optional social convention. Sheldon relents, and Penny takes him to shop for a gift. Howard is sent to get Leonard out of the apartment for a couple of hours so they can set up the party.
Back at the apartment, Leonard is playing Halo. Howard invites Leonard to a special showing of Blade Runner with 8 extra seconds of footage, but Leonard is intent on beating a longtime rival from Copenhagen. As a last ditch effort, Howard fakes eating a granola bar with peanuts in it and having an allergic reaction. Leonard drops his game and rushes him to the hospital. Howard tries to ask the ER nurse to play along and keep him under observation for an hour or so, to which she replies "this hospital is not equipped to treat stupid". Howard calls Penny to tell her that he can't delay Leonard any longer. Penny puts the pressure on, promising Howard that if he succeeds in keeping Leonard out of the apartment until they're done, she will clue him in to which of her friends are easy...one with self esteem issues, one who punishes her father by sleeping around, and one who is two shots away from letting him "wear her like a hat". Howard says "I'm doing this for you, little buddy" to his crotch and eats the peanut-riddled granola bar, creating an actual medical crisis as he swells up.At the mall, Penny tries to help Sheldon choose a gift. Sheldon is leaning towards something practical, Penny tries to encourage him to buy something "fun". Sheldon explains how when he was 12 he wanted a titanium centrifuge, but his parents got him a motorized dirt bike, as if ANY 12 year old boy would want such a thing. Sheldon settles on a router for Leonard's birthday gift. Overhearing Sheldon's explanation of the merits of the gift, several customers in the store ask Sheldon for advice on their own purchases. Eventually, Sheldon is servicing customers like an employee, even using the store's computer to place special orders for them. As Penny drags him from the store, he berates the store employees that "1 2 3 4" is not an adequate password for their computer system.
Back at the hospital, Howard has swelled up like a puffer fish. He is treated and released.
When they finally arrive back at the apartment, the party is already over. Raj is drunk, shirtless, and in mid-Karoke, singing Cindi Lauper's "True Colors". Penny and Sheldon are asleep, crashed in the living area. Penny wakes up and shares videos taken during the party with Leonard and gives Leonard a birthday kiss. Leonard chases after her, asking about her birthday date.
- Guest starring:
- Teleplay: David Goetsch & Steven Molaro
- Story: Lee Aronsohn & Bill Prady
- "The show at its best. The characters are clear and the humor is consistent. A really enjoyable episode." - The TV Critic's Review
- IMDb user reviews
- Title Reference: The allergic reaction that Howard experienced when he ate the Granola Bar containing peanuts.
- Chuck Lorre's vanity card 
- This episode was watched by 7.79 million people with a rating of 2.9 (adults 18-49).
- Episode transcript 
- Until "The Intimacy Acceleration", this was the only episode in the entire series where a main character's birthday is celebrated.
- Sheldon's statement about horoscopes being hokum is based on the work of Bertram Forer, a psychologist from UCLA. His classic 1948 experiment involved administering personality tests to groups of subjects, and then giving each subject the same exact personality evaluation (copied from a newspaper astrology column) and asking them to evaluate the accuracy of the results on a scale of 1 to 5. The average accuracy was rated at an incredible 4.2, indicating that people are generally willing to accept generalized descriptions of themselves because they WANT them to be true. This experiment has been repeated hundreds of times by various researchers; always with almost exact results, giving us "The Forer Effect" or "subjective validation".
- As Howard attempts to remove Leonard from the apartment, in several camera angles, you can see the back of the Xbox is clearly not plugged in.
- When Howard attempts to bribe the ER nurse, Althea, with "the man who freed your people", the bill he's offering must be one with Abraham Lincoln on it, which would be $5. Her response to his bribe indicates she'd need to see six bills with Benjamin Franklin on them to be bribed. Benjamin Franklin is on the $100 bill, so Howard is about $595 short.
- Penny kisses Leonard.
- The ER nurse, Althea, is the same person (or at least the same actress) as the waiting room nurse in the sperm bank from the "Pilot". According to her nametag, her surname is Davis.
- Howard`s reaction is created with clever synthetics on the actor who plays him.
- The secret knock Sheldon demonstrates to Howard and Raj has the same pattern as the secret knock Leon teaches Mathilda in the 1994 French thriller, "Leon: The Professional".
- Sheldon says that if Leonard was deprived of Christmas, he would be better as an experimental physicist; however, it is shown "The Maternal Congruence" that Leonard never really had fond memories of Christmas as a child either. This statement is kinda excusable due to the fact that Sheldon thinks of Leonard's Christmas to actually be fun.
- In the "Boompa Loved His Hookers" episode of Two and a Half Men, Alan Harper tried to bribe airline employee Karen played by Vernee Watson with five dollars (a ten with a request for five dollars in change) just like Howard did in this episode. Both shows are Chuck Lorre productions.
- Penny, while Sheldon explains why horoscopes are fake, says "Blah blah blah, typical Taurus", indicating Sheldon's birthday is 19 April to 18 May. However, in The Intimacy Acceleration, she states that she "never would have had you [Sheldon] as a Pisces". This is explained by the fact Penny probably guessed he was Taurus as Sheldon wouldn't have told her his birthday.
- Leonard: How did you know my birthday's Saturday?
- Penny: I did your horoscope remember? I was going to do everybody's until Sheldon went on one of his typical psychotic rants.
- Sheldon: For the record, that psychotic rant was a concise summation of the research of Bertram Forer, who, in 1948, proved conclusively through meticulously designed experiments, that astrology is nothing, but pseudo-scientific hokum.
- Penny: Blah, blah, blah, a typical Taurus. So, seriously, we gonna see you Saturday?
- Leonard: Uh, I don't think so.
- Penny: Why not?
- Leonard: I don't celebrate my birthday.
- Penny: Shut up, yeah, you do.
- Leonard: It's no big deal; it's just the way I was raised. My parents focused on celebrating achievements, and being expelled from a birth canal was not considered one of them.
- Penny: Uh, that's so silly.
- Sheldon: It's actually based on very sound theories; his mother published a paper on it.
- Penny: Well what was it called, I Hate My Son and That's Why He Can't Have Cake?
- Sheldon: It was obviously effective, Leonard grew up to be an experimental physicist, perhaps if she also denied him Christmas, he'd be a little better at it.
- Leonard (sarcastically to Sheldon): Thank you.
- Howard: Well, I love birthdays; waking up to Mom's special French Toast breakfast, wearing the birthday king crown, playing laser tag with all my friends...
- Penny: Yeah! See? That's what kids should have!
- Howard: Actually, that was last year.
- Penny: Okay, here’s the deal, you either help me throw Leonard a birthday party or, so help me God, I will go into your bedroom and I will unbag all of your most valuable mint condition comic books. And on one of them, you won’t know which; I’ll draw a tiny happy face in ink.
- Sheldon: You can’t do that, if you make a mark on a mint comic book it’s no longer mint.
- Penny: Sheldon, do you understand the concept of blackmail?
- Penny: Okay, how about this. You keep him there a little longer and when you get to the party I’ll point out which of my friends are easy.
- Howard: Don’t toy with me, woman.
- Penny: I’ve got a hot former fat girl with no self-esteem, I've got a girl who punishes her father by sleeping around and an alcoholic who’s two tequila shots away from letting you wear her like a hat.
- Howard: Thy will be done. (Thinks. Reaches into back pocket, finds the half a granola bar from earlier. Looks down.) I’m doing this for you, little buddy. (Takes a bite.)
- Leonard: Excuse me, my friend is having an allergic reaction to peanuts!
- Nurse Althea: No, he's not.
- Leonard: (concerned) Yes, he is!
- Nurse Althea: Look, sir we are very busy here and I just don’t… (sees Howard's peanut allergic face) Holy Crap! [Reaction]
- Howard: (swollen and mumbling) Please help me.
- Nurse: (on the intercom) Code 4, I need a gurney, right away, right away.
- Howard: Thank you.