Anatomy III: Immune System Overview

Anatomy III: Blood and the Cardiovascular and Respiratory Systems

• This is a course on the anatomy of the blood, cardiovascular, and respiratory systems.

Lesson 28: Anatomy of the Immune System. Generalities

• The immune system is a system for protection against diseases.
• The immune system distinguishes normal body cells from pathogens like bacteria, viruses, and cancer cells.
• The immune system is classified as innate and adaptive, and further divided into humoral and cellular components.
• The upcoming lessons will cover the anatomical basis of the immune system, excluding the physiological workings.

Composition of Lymphoid System

• Lymphoid organs produce lymphocytes.
• Primary lymphoid organs (where immature lymphocytes develop):
  • Thymus
  • Red bone marrow
• Secondary lymphoid organs (where lymphocytes remain until antigen presentation):
  • Spleen
  • Tonsils
  • Lymph nodes
  • Some lymphoid nodules in the alimentary tract and appendix

Thymus

• Located in the anterior and superior mediastinum.
• Shape: flat triangle.
• After puberty, the thymus degrades into fat tissue.

Bone Marrow

• Main hematopoietic organ and primary lymphoid organ.
• Produces 500 billion blood cells per day.
• Generates lymphocytes from immature hematopoietic progenitor cells.
• Transplantable, used in cancer treatment (e.g., multiple myeloma, leukemia).
• Contains two types of bone marrow:
  • Yellow marrow: fat cells in the medullary cavity.
  • Red marrow: hematopoietic tissue, where red blood cells, platelets, and lymphocytes develop.
• At birth, all bone marrow is red; with age, it becomes yellow.
• Yellow marrow can convert to red marrow during severe hypovolemia (low blood volume).
• Major locations include the pelvis, vertebrae, sternum, ribs, cranium, scapulae, and the spongy ends of long bones (e.g., humerus, femur).

Spleen

• Located in the left upper quadrant of the abdomen (hypocondrium).
• Shape: purple fist; largest lymphatic organ.
• Most vulnerable abdominal organ.
• Lymphoid organ due to:
  • Lymphocyte proliferation
  • Immune surveillance
  • Immune response.
• Recycles iron and globin from old blood cells and platelets.
• Acts as a blood reservoir, storing red blood cells and platelets.
• Not a vital organ; survival is possible without it.
• Capable of expansion and contraction.

Tonsils

• The pharynx has abundant lymphoid tissue forming tonsils.
• This lymphoid forms a tonsillar ring around the superior part of the pharynx (Waldeyer's ring).
• Protects from external bodies entering the body through the mouth and nose.

Pharyngeal Tonsil (Adenoids)

• Located at the nasopharynx's mucous membrane.
• Can cause dysphagia (difficulty swallowing) if enlarged.
• Separated from the superior pharyngeal constrictor muscle by a capsule.

Tubal Tonsils

• Lymphoid tissue in the pharynx's submucosa.
• Near the nasopharyngeal opening to the pharyngotympanic (Eustachian) tube.

Palatine Tonsils

• Bilateral collections of lymphoid tissue.
• Located between palatine arches.
• Removal is called tonsillectomy.
• Tonsillar fossa is located between the palatoglossal and palatopharyngeal arches.
• Tonsillar bed is formed by the superior pharyngeal constrictor muscle and the thin fibrous sheet of the pharyngobasilar fascia.

Lingual Tonsils

• Located on the dorsal surface of the base of the tongue.
• Partially encapsulated.
• Has many associated mucous glands.

Mucosa-Associated Lymphoid Tissue (MALT)

• Small groups of lymphoid tissue found in various body sites.
• Formed by T cells, B cells, plasma cells, and macrophages.
• Antigens from mucosal epithelium stimulate immune response.
• Microfold cells (M cells) transport antigens from the gut lumen into lymphoid tissue.
• Examples include BALT (bronchus-associated lymphoid tissue), GALT (gut-associated lymphoid tissue), and SALT (skin-associated lymphoid tissue).

Peyer's Patches

• Named after Johann Conrad Peyer.
• 1 cm-sized lymphoid tissue in the ileum.
• Observable by microscope as oval or round lymphoid follicles.
• Found in the lamina propria layer of the mucosa, extending into the submucosa.
• Microfold (M) cells are present between the lumen and the follicle.
• B lymphocytes are in the follicle center.
• T lymphocytes are between the follicles.
• Activated lymphocytes enter the blood via the thoracic duct.

Microfold Cells

• Are antigen-presenting cells.
• Phagocytose antigens from the gut lumen.
• Deliver antigens via transcytosis to dendritic cells, macrophages, and T lymphocytes.

Vermiform Appendix

• Blind intestinal diverticulum containing lymphoid tissue.
• Approximately 10 cm long.
• Arises from the cecum inferior to the ileocecal junction.
• Position is variable, most commonly retrocecal.