What is the anatomy, blood supply, venous drainage, lymphatic drainage, innervation, and function of the adenoids (pharyngeal tonsil)?

Adenoid Anatomy

Location and Structure

The adenoids (pharyngeal tonsils) are masses of lymphoid tissue located at the junction of the roof and posterior wall of the nasopharynx, positioned ideally to sample antigens passing through the nasal and oral cavities. 1, 2

• The adenoids form part of Waldeyer's ring of lymphoid tissue, serving as the uppermost component of this protective immunological barrier 1, 3
• They are strategically positioned adjacent to the auditory tube openings and choanae (posterior nasal openings) 4
• The tissue consists of a pad of lymphoid material on the posterior pharyngeal wall, with growth progressing in an anterior and inferior direction during childhood 2

Blood Supply

The adenoids receive arterial blood from branches of the external carotid artery system, primarily through the facial and maxillary arteries, with additional contributions from lingual artery branches and pharyngeal branches. 5

• Understanding this vascular anatomy is clinically critical because bleeding complications occur in approximately 2.65% of adenoidectomy procedures 5
• About one-third of tonsillectomy/adenoidectomy-related deaths are attributable to hemorrhage 5
• Patients with bleeding diatheses, major heart disease, or certain craniofacial disorders face higher bleeding risks during surgery 5

Venous Drainage and Lymphatic System

While the provided evidence does not explicitly detail venous drainage pathways, the adenoids function as secondary lymphoid organs with extensive lymphatic connections 6:

• The tissue contains well-defined microcompartments including the cryptepithelium, follicular germinal centers with mantle zones, and interfollicular areas 3
• These structures facilitate the generation and dissemination of antigen-specific memory B-lymphocytes throughout the upper respiratory tract 3

Innervation

The provided evidence does not contain specific information about adenoid innervation patterns.

Microscopic Structure and Cellular Organization

The adenoid epithelium is specialized reticular epithelium containing M-cells that actively transport antigens into underlying lymphoid tissue for immune processing. 6, 3

• M-cells in the cryptepithelium take up antigens into vesicles and transport them to extrafollicular regions or lymphoid follicles 6
• The reticular epithelium overlying lymphoid follicles contains significantly more B-cells, CD4+ cells, and CD11c+ cells compared to non-reticular epithelium 7
• Interdigitating dendritic cells and macrophages process antigens and present them to helper T lymphocytes in the extrafollicular region 6

Immunological Function

The adenoids serve as the first line of immune defense against respiratory and digestive antigens, with peak immunologic activity occurring between ages 3 and 10 years. 6, 1

• The tissue produces all five immunoglobulin isotypes, with IgA being the most important for mucosal immunity 6
• B lymphocytes develop into either antibody-expressing memory cells capable of migrating to the nasopharynx or plasma cells that produce antibodies 6
• The adenoids produce immunocytes bearing the J (joining) chain carbohydrate, necessary for forming secretory IgA (SIgA), a critical component of upper airway mucosal immunity 6

Age-Related Changes

Adenoid tissue demonstrates predictable growth and involution patterns, with peak size occurring between ages 4-8 years, followed by progressive involution. 2

• Adenoid depth reaches peak growth at age 4 years 2
• Adenoid height and thickness peak at 8 years of age 2
• Involution occurs in a posterior and superior direction, beginning first with adenoid depth, followed by height and thickness 2
• The tissue is most prominent during childhood (ages 3-10 years) and subsequently demonstrates age-dependent involution 6

Clinical Pitfalls

• Do not assume adenoid size correlates directly with obstruction severity—nasopharyngolaryngoscopy provides more accurate assessment of airway obstruction than radiological evaluation alone 4
• Children under 3 years of age have higher complication rates during adenoidectomy procedures 5
• Patients with Down syndrome, cerebral palsy, neuromuscular diseases, or craniofacial disorders face increased surgical risks 5