Some, like the smallpox vaccine on one exposure generates a very high response that lasts for many years so you don't need a boost.

有一些疫苗,比如接触一次天花疫苗后,人体就能产生高水平的免疫应答,并持续很多年,所以无需再次接种

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Because on second exposure there's another population of cells that I haven't mentioned before that remain after the primary exposure and those are called memory cells, they're down here.

因为在再次免疫中,有另外一些细胞,我之前并没有提到过它们,在初次免疫结束后保留下来的细胞,被称为记忆细胞,它们在下面

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It's the way, for example, that foreign molecules or antigens get presented to cells of your immune system in order to start the process of making an immune response, so sometimes a cell/cell interaction.

通过这种方式,外源分子,或者说抗原,进入到免疫系统的细胞中后,就会开启免疫反应过程,有时候就是细胞之间的相互作用

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Now, if I was a person that was designing a vaccine and I noticed that this was the response that it got, that antibodies were produced, they reached some intermediate level, they started to fall, I would say, "Well I haven't stimulated the immune system enough, let me re-boost, let me give another dose of antigen."

现在,如果我正在研究一种疫苗,我注意到机体会对这种疫苗产生应答,也就是会产生抗体,当抗体浓度达到中等水平后,就开始下降,就可以说,疫苗对免疫系统刺激还不足,我需要提高抗原浓度,让我再把剂量再加大一倍

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What happens when they're activated is they start to reproduce, they make more and more, and more B-cells.

激活后,免疫细胞开始增殖,B细胞变得越来越多

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If you looked later, as the antibody production response matures, some IgG is produced so that in the late period after initial priming you'd have a mixture of IgM and IgG in the blood.

如果你稍后再观察,等到产生抗体的应答趋于成熟后,就会产生一些IgG抗体,在初次免疫的最终阶段,你的血液中有IgM,和IgG两种抗体

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