Reactive lymphadenopathy: This is the most common cause of swollen neck, often due to a viral or bacterial infection, which leads to the enlargement of lymph nodes as the body responds to the infection.

Tonsillitis: Inflammation of the tonsils can cause swelling in the neck due to the enlargement of lymph nodes in response to the infection.
Mononucleosis (Mono): Caused by the Epstein-Barr virus, mono can lead to swollen lymph nodes in the neck, among other symptoms like fever and sore throat.
Dental infections: Abscesses or infections in the teeth or gums can cause swelling in the neck due to the spread of infection to the lymph nodes.
Thyroiditis: Inflammation of the thyroid gland can cause swelling in the lower part of the neck.

Lymphoma: Although less common, lymphoma (cancer of the lymphatic system) can cause swollen lymph nodes in the neck and is critical to diagnose early.
Metastatic cancer: Cancer that has spread to the lymph nodes in the neck from another part of the body can cause swelling and is crucial to identify for appropriate treatment.
Tuberculosis (TB): TB can cause swollen lymph nodes, especially in the neck, and is important to diagnose due to its infectious nature and the need for specific treatment.
Abscess or infected cyst: A collection of pus in the neck can cause severe swelling and requires prompt drainage to prevent complications.

Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the lymph nodes in the neck, leading to swelling.
Kawasaki disease: A rare condition that mainly affects children, causing inflammation in the walls of medium-sized arteries throughout the body, and can lead to swollen lymph nodes in the neck.
Castleman disease: A rare disorder that involves an overgrowth of cells in the body's lymph nodes, which can cause swelling in the neck.
Cystic hygroma: A rare congenital condition that can cause swelling in the neck due to a fluid-filled sac.