Introduction Secondary lymphoid tissue has many important roles in immunity including filtration and sampling of bodily fluids in order to detect infection sequestration of pathogens and ingestion by innate immune cells activation of adaptive immunity by antigen presenting cells maintainance of immunological memory by storage of long lived cells There are several types of secondary lymphoid tissue including the spleen a solid organ in the left upper quadrant of the abdomen lymph nodes a diffuse network of small nodes throughout the body mucosal lymphoid tissue patches of immunological tissue such as Waldeyer's ring consisting of tubular tonsils in the pharyngeal recess palatine tonsils lingual tonsils at the base of the tongue Peyer patches in the lower jejunum and ileum Spleen The spleen is located in the left upper quadrant of the abdomen where it is protected by the 9th to 11th ribs it sits anterior to the left kidney There are several components of the spleen, which are divided into white pulp, which is the the immunologically active part of the spleen containing germinal centers containing activated B-cells (part of a larger B-cell follicle) periarteriolar lymphatic sheaths (PALS) containing T-cells marginal zones containing macrophages and other antigen-presenting cells red pulp, which is the hematologically active part of the spleen characterized by arterioles to allow for delivery of red blood cells (RBCs) resident macrophages to ingest both dying/abnormal RBCs and encapsulated bacteria sinusoids with a fenestrated basement membrane to allow for red RBC exit There are many clinical manifestations of asplenia including increased susceptibility to infections by encapsulated bacteria appearance of abnormal RBC forms on peripheral blood smear others described more fully in the asplenia topic Lymph Nodes Lymph nodes are a diffuse network of encapsulated lymphoid tissue that have many regional afferent lymphatic vessels have at least one efferent lymphatic There are several components of a lymph node, which are divided intothe cortex an outermost layer of the lymph node that is composed of follicles composed of proliferating B cells that can be dormant and cell dense primary follicles active and pale secondary follicles germinal centers where isotype switching occurs underdeveloped in patients with X-linked agammaglobulinemia subcapsular macrophages that monitor incoming lymph dendritic cells that serve as antigen presenting cells paracortex an intermediate layer of the lymph node that is composed of high endothelial vessels (HEVs) where lymphocytes enter lymph nodes T-cells that await activation, meaning that this region is hypertrophied in infections triggering cellular immunity underdeveloped in patients with DiGeorge syndrome medulla an innermost layer of the lymph node that is composed of cords with closely packed lymphocyte and plasma cell "cords" sinuses that drain into the efferent lymph vessels and house reticular cells macrophages Lymph Node Drainage Pattern Lymph Node Drainage Pattern Lymph Node Cluster Drainage Area Cervical Head Neck Hilar Lungs Mediastinal Trachea Esophagus Axillary Upper limb Breast Skin above the umbilicus Celiac Liver Stomach Spleen Pancreas Upper duodenum Superior mesenteric Lower duodenum Jejunum Ileum Colon proximal to splenic flexure Inferior mesenteric Colon between splenic flexure and upper rectum Internal iliac Lower rectum Anal canal proximal to pectinate line Bladder Cervix Prostate Paraaortic Testes Ovary Kidney Uterus Superficial inguinal Anal canal distal to pectinate line Skin below the umbilicus Scrotum Vulva Popliteal Dorsolateral foot Posterior calf