Which sample source provides the greatest number of B cells?

The spleen is the correct choice for providing the greatest number of B cells. This organ plays a crucial role in the immune system, specifically in managing blood-borne pathogens. The spleen contains various areas, such as the white pulp, which is a site rich in lymphoid tissue where B cells are produced and mature.

In the spleen, B cells can encounter antigens and undergo activation, leading to proliferation and differentiation into antibody-secreting plasma cells. This makes the spleen a significant reservoir of B cells, as it not only produces them but also acts as a site for their activation and function.

While other sources, such as whole blood, thymus, and lymph nodes, contain immune cells, they do not provide B cells in the same capacity as the spleen. Whole blood contains a mixture of all cell types in circulation, largely comprising T cells and other leukocytes with B cells present only at a much lower concentration. The thymus is primarily the site for T cell development, and lymph nodes are an important site for immune responses but typically have fewer B cells than the spleen itself.