Four stages that take place in the primary immune response to a pathogen are given below. P B-lymphocytes divide by mitosis. Q T-helper cells interact with macrophages that have digested pathogens. R Plasma cells secrete antibodies. S T-helper cells activate other lymphocytes. These stages may be ordered to show the sequence in which they happen. Which stage is third in the sequence?
- AP
- BQ
- CR
- DS