Question

*The answer is D.* B and T lymphocytes circulate via the vascular system to secondary lymphoid organs and tissues. Included among these tissues are lymph nodes, mucosa-associated lymphoid tissues, and spleen. In the case of lymph nodes, lymphocyte trafficking occurs through specialized postcapillary venules termed high endothelial venules (HEVs). HEVs express an array of specific cell adhesion molecules (e.g., CD31) that allow lymphocyte binding and diapedesis. The cuboidal shape of HEV cells reduces flow-mediated shear forces and specialized intercellular connections facilitate egress of lymphocytes out of the vascular space. Afferent and efferent lymphatic channels (choices A and B) do not possess HEVs. Hassall corpuscles (choice C) are found in the medulla of the thymus. Peyer patches (choice E) are organized lymphoid tissues found in the small intestine.

Diagnosis: Lymphadenopathy, streptococcal pharyngitis

Answer 1

The lymphatic system is responsible for transporting and maturing lymphocytes, with primary lymphoid organs being the bone marrow and thymus. Secondary lymphoid organs like spleen, lymph nodes, and tonsils filter lymph and house immune cells. Lymph nodes contain germinal centers and HEVs, critical for lymphocyte trafficking and initiating adaptive immune responses.

Step-by-step explanation:

Understanding the Lymphatic System and Lymphocyte Circulation

The lymphatic system is a crucial component of the immune response, facilitating the transport of lymphocytes as well as the filtration of foreign particles. The primary lymphoid tissues, such as the bone marrow and thymus, are where lymphocytes mature. B cells mature in the bone marrow, while T cells mature in the thymus.

Secondary lymphoid organs include structures like the spleen, lymph nodes, and tonsils, which serve as meeting points for mature lymphocytes to initiate an adaptive immune response. Lymph nodes, scattered throughout the body, are pivotal as they filter lymph fluid gathered from tissues. These lymph nodes harbor germinal centers filled with B and T lymphocytes, dendritic cells, and macrophages. Lymph enters these nodes through the afferent lymphatic vessels, interacts with immune cells within the germinal centers, and exits through the efferent lymphatic vessels.

Within the lymph nodes, high endothelial venules (HEVs) are specialized postcapillary vessels that enable lymphocytes to leave the bloodstream and enter the lymphoid tissues. These HEVs are lined with thicker and more columnar endothelial cells compared to normal vessels. Antigen-presenting cells (APCs) within lymph nodes process antigens and play a critical role in alerting nearby lymphocytes about potential pathogens, thereby facilitating an effective immune response.