摘要
The host immune system is comprised of many unique cell types that respond to pathogen-derived molecules （foreign antigens） and play key roles in combating infection. Immune cells have been historically classified into two groups. Innate immune ceils, such as macrophages and dendritic cells, recognize biochemical patterns within compounds or proteins expressed by microorganisms, and upon activation secrete proinflammatory cytokines im- mediately or within hours following infection. On the other hand, adaptive immune cells, such as B and T cells, react with exquisite specificity to small protein determinants encoded by patho- gens and clonally expand in numbers over the course of seven to ten days following infection or vaccination.
