Sperm cells can be seen as a haploid phase of many organisms' lifecycle, so are alive by pretty much any definition. In humans, the haploid phase of the lifecy

Sperm cells can be seen as a haploid phase of many organisms' lifecycle, so are alive by pretty much any definition. In humans, the haploid phase of the lifecycle is single-celled, while the diploid phase is multicellular. In contrast, in mosses and fungi the haploid phase is multicellular while the diploid phase (sporophytes/zygote) is single-celled. Red blood cells are discussed in the article.