What type of cells mature in the spleen?

Lymphocytes are the type of cells that mature in the spleen. The spleen plays a crucial role in the immune system, where it acts as a site for the maturation and proliferation of lymphocytes, which are essential for immune responses. There are two main types of lymphocytes: B cells and T cells. The spleen's environment supports the differentiation of these cells, enabling them to become fully functional in protecting the body against pathogens.

Red blood cells do not mature in the spleen; rather, they are produced in the bone marrow. Platelets are fragments of cells that originate from megakaryocytes in the bone marrow as well and do not undergo maturation in the spleen. Epithelial cells line surfaces and cavities of structures throughout the body and are not specifically associated with the maturation process in the spleen. Therefore, lymphocytes distinctly stand out as the key cells that mature in the spleen, making this the correct answer.