Tuberculosis (TB): This is a common cause of cervical lymphadenitis, and spleen involvement can occur in disseminated TB. The combination of lymph node enlargement and spleen masses, especially in endemic areas or in individuals with risk factors for TB, makes this a leading consideration.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can present with cervical lymphadenopathy and splenomegaly (enlargement of the spleen) due to lymphomatous involvement. The presence of systemic symptoms such as fever, night sweats, and weight loss would support this diagnosis.
Infectious Mononucleosis: Caused by Epstein-Barr virus (EBV), this condition can lead to cervical lymphadenitis and splenomegaly. It is more common in young adults and often presents with fever, sore throat, and fatigue.
Toxoplasmosis: In immunocompromised patients, toxoplasmosis can cause lymphadenopathy and splenic enlargement. This diagnosis would be considered, especially if there's a history of exposure to cats or undercooked meat.

HIV Infection: HIV can present with persistent generalized lymphadenopathy and may involve the spleen. Given the significant implications for treatment and prognosis, missing this diagnosis could be detrimental.
Cat-Scratch Disease: Caused by Bartonella henselae, this condition typically presents with lymphadenopathy near the site of the cat scratch. While less common, it can cause systemic symptoms and should be considered to avoid missing a treatable condition.
Malignant Histiocytosis: A rare but aggressive condition that can present with lymphadenopathy and splenomegaly. It's crucial to consider this diagnosis due to its poor prognosis if left untreated.

Castleman Disease: A rare disorder that can cause lymphadenopathy and may involve the spleen. It exists in unicentric and multicentric forms, with the latter being more systemic and potentially involving the spleen.
Sarcoidosis: While more commonly associated with pulmonary symptoms, sarcoidosis can cause lymphadenopathy and, less frequently, splenic involvement. It would be considered in the differential, especially if there are other systemic symptoms or findings suggestive of sarcoidosis.
Brucellosis: A zoonotic infection that can cause lymphadenopathy, splenomegaly, and systemic symptoms. It's rare in many parts of the world but should be considered in individuals with exposure to livestock or consumption of unpasteurized dairy products.