Which of the following is included in secondary lymphoid tissue?

Secondary lymphoid tissues are essential components of the immune system where immune responses are initiated and where lymphocytes can interact with antigens. Peyer's patches are a prime example of secondary lymphoid tissue as they are small masses of lymphatic tissue found in the wall of the small intestine. They play a crucial role in monitoring intestinal bacteria and preventing the growth of pathogenic bacteria in the intestines.

In contrast, bone marrow and the thymus are classified as primary lymphoid organs. The bone marrow is where all blood cells, including immune cells, are produced, while the thymus is where T cells mature. Since the question specifically inquires about secondary lymphoid tissues, Peyer's patches are correctly identified as part of this category, highlighting their importance in the immune response at mucosal surfaces.