What are the definitions of the underlined “it”, please?

[Bhikkhu1991a]

Hello,

What are the definitions of the following underlined “it”, please? What are their antecedents, for example?

Here are the excerpts from the Newsletter “A bit of a laugh” dated November 14, 2008 (http://www.english-test.net/newsletter/a-bit-of-a-laugh-175.html):

1. It mightn't strike you as at all funny but then if it doesn't, at least I shall be spared the embarrassment of you not reacting at all. (Paragraph 1, line 4)

2. Of course you can laugh so much that you simply can't stop. You know what it's like, it's catching. (Paragraph 4, line 2)

3. You want to laugh but you can't and so you keep it to yourself and laugh up your sleeve — I should imagine if you tried doing this, it could be very awkward! (Paragraph 4, line 8 )

4. Mind you, it could be that you and the others get jobs somewhere else for much more money and then it's the turn of the boss to laugh on the other side of his face. And of course nowadays getting a job is important and it's a very serious business if you can't get one or you lose your present job. (Paragraph 4, line 12)

I hope to hear from you soon. Thank you so much.

Best wishes.