MORPHOLOGYInflamednodesinacutenonspecificlymphadenitisareswollen,gray-red,andengorged.Histologically,therearelarge germinal centers containingnumerousmitoticfigures.Whenthecauseisapyogenicorganism,aneutrophilicinfiltrateisseenaroundthefolliclesandwithinthelymphoidsinuses.Withsevereinfections,thecentersoffolliclescanundergonecrosis,leadingtotheformationofanabscess.
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Affectednodesaretenderandmaybecomefluctuantifabscessformationisextensive.Theoverlyingskinisfrequentlyredandmaydevelopdrainingsinuses.Withcontroloftheinfectionthelymphnodesmayreverttoanormal“resting”appearanceor,ifdamaged,undergoscarring.
Depending on the causative agent, chronic nonspecific lymphadenitis can assume one of three patterns: follicular hyperplasia, paracortical hyperplasia, or sinus histiocytosis.
MORPHOLOGYFollicular Hyperplasia. ThispatternoccurswithinfectionsorinflammatoryprocessesthatactivateBcells,whichmigrateintoBcellfolliclesandcreatethefollicular (or germinal center) reaction. ReactivefolliclescontainnumerousactivatedBcells,scatteredTcells,phagocyticmacrophagescontainingnucleardebris(tingiblebodymacrophages),andameshworkofantigen-presentingfolliculardendriticcells.Causesoffollicularhyperplasiaincluderheumatoid arthritis, toxoplasmosis, andearlyHIV infection. Thisformoflymphadenitismustbedistinguishedfromfollicularlymphoma(discussedlater).Findingsthatfavorfollicularhyperplasiaare(1)thepreservationofthelymphnodearchitecture;(2)variationintheshapeandsizeofthegerminalcenters;(3)thepresenceofamixtureofgerminalcenterlymphocytesofvaryingshapesandsizes;and(4)prominentphagocyticandmitoticactivityingerminalcenters.
Paracortical Hyperplasia. ThispatterniscausedbyimmunereactionsinvolvingtheT cell regions ofthelymphnode.Whenactivated,parafollicularTcellstransformintolargeproliferatingimmunoblaststhatcaneffacetheBcellfollicles.Paracorticalhyperplasiaisencounteredinviral infections (suchasEBV),aftercertainvaccinations (e.g.,smallpox),andinimmunereactionsinducedbydrugs (especiallyphenytoin).
Sinus Histiocytosis. Thisreactivepatternischaracterizedbydistentionandprominenceofthelymphaticsinusoids,owingtoamarkedhypertrophy of lining endothelial cells andaninfiltrate of macrophages (histiocytes). Itoftenisencounteredinlymphnodesdrainingcancersandmayrepresentanimmuneresponsetothetumororitsproducts
Cat-scratch disease is a self-limited lymphadenitis caused by the bacterium Bartonella henselae. It is primarily a disease of childhood; 90% of the patients are younger than 18 years of age. It manifests with regional lymphadenopathy, most frequently in the axilla and the neck. The nodal enlargement appears approximately 2 weeks after a feline scratch or, less commonly, after a splinter or thorn injury. An inflammatory nodule, vesicle, or eschar is sometimes visible at the site of the skin injury. In most patients the lymph node enlargement regresses during a period of 2 to 4 months. Rarely, encephalitis, osteomyelitis, or thrombocytopenia may develop in patients.