How do memory cells(B-cells) encounter pathogens? From what I understand, once the infection is handled, some of the B cells capable of producing the correct antigens are stored for the long-term in the lymph nodes. They will start multiplying again upon encountering the pathogen again. What I don't understand is how can these cells, that should be in relatively small numbers, can effectively encounter the pathogen ? Do all pathogens travel to the lymph nodes? Do the memory B cells travel out and encounter pathogens?

Great Question.

**Location:** Memory B cells are highly abundant in the human spleen, and they make up 45% of the total B cell population in this organ. There is evidence to indicate that human memory B cells might mostly reside in the spleen, and some memory B cells recirculate in the blood [1].

**Activation:**

Memory B cells circulate throughout the body in a quiescent state until specific antigen is re-encountered and triggers a potent secondary immune response [2].T cell help is a strict requirement for the reactivation of memory B cells that are specific for monomeric protein antigens [3].

Sources:

1. Hauser, Anja E., and Uta E. Höpken. "B cell localization and migration in health and disease." Molecular Biology of B Cells (Second Edition). 2015. 187-214.

2. Firestein, Gary S., et al. Kelley's Textbook of Rheumatology E-Book. Elsevier Health Sciences, 2012.

3. Janeway, Charles A., et al. "Immunobiology: the immune system in health and disease." (2005).