I have read that if you tough strep throat out your body could build up natural immunity. what are the pros and cons of not treating strep throat and likelihood of contracting other infections ? also how likely is it that you would build immunity?

Strep: If getting strep at your age, you must still have tonsils. If having 3-4 strep infections per year or if strep interfering with work or personal life, time to consider tonsillectomy. Immunity issue probably not applicable, unless you are immune deficient.

Treat strep!: You don't want rheumatic heart disease. There are several species of strep After you have one species, it is true that you are immune to that species. It does not mean that it did not give you rheumatic heart disease first. And it does not mean you are less likely to get the other species os strep. So take your antibiotic for all 10 days (Less days won't prevent rheumatic fever).