In a primary follicle, which B cell population is present?

• A. Unstimulated B cells
• B. Germinal centers
• C. Plasma cells
• D. Memory cells

Answer: (A)

In a resting, or primary, follicle the B cells are naive and unstimulated. This is the state before they've encountered antigen, so the population you’d observe is composed of these naive B cells just waiting to be activated. When an antigen is encountered, B cells migrate into a secondary follicle and form a germinal center, where they proliferate and undergo processes like somatic hypermutation and class switching. From there, cells differentiate into antibody-secreting plasma cells or into memory B cells for long-term immunity. Since those activated and differentiated populations come from the germinal center reaction, they aren’t present in a primary follicle.