How bad is drinking alcohol for my body?

I turned 21 approximately 6 months ago. Before I turned 21, I could count on one hand the number of times I had drank, and I had never been drunk before. I just felt like underage drinking was dumb, but now that I'm 21 I guess you could say that I'm taking full advantage of it...especially now that it's summer and I'm on break from school and such. How bad is drinking for me? Like, will it have serious long term effects? I usually drink maybe like once or twice a week, however, this week I'll probably be drinking like 4 days :/ When I do drink, I usually just have like 3 or 4 drinks (sometimes more), and I've never passed out or thrown up but there are definitely gaps in my memory from times when I've been drunk. I feel like once school starts again and once I'm 22 I won't be drinking as much anymore, but by drinking so much this year, will I have problems in the future? I don't want to kill my brain cells, I need them!