Cells constantly swap cargo bound in vesicles, miniscule membrane-enclosed packages of proteins and other chemicals. Before the swap can take place, the vesicle membrane must fuse with another membrane, creating channels packages can pass through.
This process, known as membrane fusion, is fundamental to health and disease. It occurs at fertilization and is particularly critical to keep hormones circulating and brain cells firing. Membrane fusion is also how HIV and other viruses infect cells.