Lymph nodes in which lymphocytes are responding to antigens often enlarge, refecting formation of germinal centers and proliferation of lymphocytes. This phenomenon is often seen in the lymph nodes of the neck in response to nasal or oropharyngeal infection, and in axillary and inguinal regions because of infection in extremities. Lymphadenitis, a reactive (infammatory) lymph node enlargement, is a common complication of microbial infections. These enlarged lymph nodes are commonly referred to as swollen glands (see Folder 14.4).
 FOLDER 14.4 Clinical Correlation: Reactive (Inflammatory) Lymphadenitis
Reactive (inﬂammatory) lymphadenitis refers to enlargement of the lymph nodes that is often secondary to bacterial and other microbial infections. Lymph nodes enlarge because of edema and hyperplasia of lymphatic nodules and their cellular components (Fig. F14.4.1). These include B lymphocytes, T lymphocytes, macrophages, and other antigen-presenting cells. In addition, infiltration of lymphatic sinuses by neutrophils is also prominent. In severe bacterial infections, lymphadenitis may be accompanied by lymphangitis, an inflammation of afferent lymphatic vessels that carry infected lymph into regional lymph nodes. Inflamed lymph vessels may be visible as red streaks under the skin in the affected area of lymphatic drainage.
Common symptoms of acute lymphadenitis are swollen lymph nodes that are tender to palpation, fever, chills, loss of appetite, tachycardia, and general weakness. Lymph
FIGURE F14.4.1 • Photomicrograph of a lymph node with nodes are usually palpable and tender, with red discol reactive lymphadenitis. Section though a superficial cortex oration on the overlying skin. In severe cases of suppura-of the lymph node shows a hyperplastic germinal center (GC) tive necrosis (necrosis with pus formation), a fistula (false projecting toward the connective tissue capsule. The majority opening) may develop that allows pus to drain from the of pale staining cells within the germinal center are enlarged lymph node to the surface. represented by B lymphocytes and macrophages;
The most common microbial organisms that cause lym-accumulation of T lymphocytes forms a distinct mantle zone phadenitis are streptococcal and staphylococcal bacteria. that encircles the germinal center. 120. (Reproduced from Other less common organisms are viruses (as in mononu-Schwarting R, McKenzie S, Rubin R. Hematopathology. cleosis or rubella), protozoa, rickettsiae, fungi, and the In: Rubin R, Strayer DS [eds]: Rubin’s Pathology: Clinicotuberculosis bacilli. Tonsillitis, infections originating from pathologic Foundations of Medicine, 5th ed. Baltimore: Lippincott Williams & Wilkins, 2008.) teeth, and bacterial pharyngitis (sore throats) are the most common causes of lymphadenitis in the neck area. Generalized lymphadenopathy is typical for rheumatoid arthritis and is detected as an early sign of HIV infection. In chronic lymphadenitis, lymph nodes are enlarged, but they are usuxally not tender.
The thymus is a lymphoepithelial organ located in the superior mediastinum.