Cells and Organs of the Immune System - Introduction to Immunology

Immune System

• The immune system is a network of cells, tissues, and organs that work together to defend the body against infections and diseases.

Cells of the Innate Immunity

• Granulocytes:
  • Neutrophils: most numerous innate immune cells (50-70% of all white blood cells), produced in bone marrow, circulate in bloodstream, and migrate to tissue to phagocytose and degrade bacteria.
  • Eosinophils: important for host defense against parasites and allergic reactions, differentiated from myeloid precursor cells in response to IL-3, IL-5, and GMCSF.
  • Basophils: involved in host defense against parasites and allergic reactions, contain toxic granules used to destroy pathogens.
• Mast cells: similar to basophilic leukocytes, found in tissues, important for defense against parasites, and mediate allergic reactions.
• Monocytes/Macrophages:
  • Found in blood and tissues, coordinate immune response, notify other immune cells of infection.
  • Present antigens to lymphocytes, have important non-immune functions (e.g., recycling dead cells).
• Natural Killer cells:
  • Phagocytic, cytotoxic cells that recognize and kill virus-infected cells or tumor cells.
  • Have receptors that recognize MHC class I molecules.
• Dendritic cells:
  • Professional antigen-presenting cells, capture, process, and present antigens to T cells.
  • Classified as classical/conventional dendritic cells (cDCs) and plasmacytoid dendritic cells (pDCs).

Cells of the Adaptive Immunity

• B cells (B lymphocytes):
  • Produce antibodies, essential for humoral immune response.
  • Mature in bone marrow (birds: Bursa of Fabricius), activated when binding antigens through their receptors (BCR).
• T cells (T lymphocytes):
  • Express antigen-specific receptor (TCR), divided into two categories: CD8+ T cells and CD4+ T cells.
  • CD8+ T cells (cytotoxic T cells): recognize and kill infected or cancer cells.
  • CD4+ T cells: divided into four subsets (Th1, Th2, Th17, and T regulatory cells).

Organs of the Immune System

• Lymph nodes:
  • Filter lymph to trap antigens, contain two important regions: outer cortex and inner medulla.
  • Contain dendritic cells that trap and process antigens, present to lymphocytes.
• Spleen:
  • Large encapsulated organ, filters blood for blood-borne antigens, cellular debris, and aged blood cells.
  • Stores red blood cells, platelets, and recycles iron.
• Mucosal-Associated Lymphoid Tissue (MALT):
  • Scattered throughout mucosal surfaces, includes Gastrointestinal-associated lymphoid tissue (GALT), Bronchial-associated lymphoid tissue (BALT), Nasal-associated lymphoid tissue (NALT), and Conjunctiva-associated lymphoid tissue (CALT).
  • GALT is the best known mucosal-associated lymphoid tissue, inductive site of intestinal immune response, and effector site of intestinal immune response.

Clinical Correlation

• The observed lymphadenopathy was lymphadenitis, a result of the bacterial infection as well as the cat's immune response to the bacterium.### Lymphoid Cells and Organs

• Lymphoid cells are first produced in the yolk sac, fetal omentum, and liver, and later in bone marrow in older fetuses and adults.

• Bone marrow is a hematopoietic organ that gives rise to all blood cells.

Primary Lymphoid Organs

• Primary lymphoid organs are where lymphocytes develop and mature.
• Examples of primary lymphoid organs include bone marrow, thymus, and Bursa of Fabricius in birds.
• Thymus is located in the thoracic cavity in front and below the heart and is large in newborns but small to invisible in adult animals.
• The thymus has a cortex with thymocytes (lymphocytes) and a medulla with few lymphocytes.
• The thymus has no lymphatic vessels leaving it.

Thymus Function

• The thymus is responsible for the maturation of T lymphocytes.
• T lymphocytes learn to recognize self-antigens in the thymus.
• Thymocytes with receptors that bind strongly to self-antigens are destroyed through apoptosis (negative selection).
• Thymocytes that cannot bind any MHC II molecules are also removed.
• Thymocytes that recognize MHC II/Ag complexes with moderate affinity are allowed to live (positive selection).

Bursa of Fabricius

• The Bursa of Fabricius is found in birds only.
• It is a primary lymphoid organ where B lymphocytes develop and mature.

Secondary Lymphoid Organs

• Secondary lymphoid organs are where lymphocytes interact with microbial antigens.
• Examples of secondary lymphoid organs include spleen, lymph nodes, non-encapsulated lymphoid tissue, and Peyer’s patches.
• Secondary lymphoid organs develop late in fetal life and persist in adult life.
• They enlarge in response to antigenic stimulation.

Lymph Nodes

• Lymph nodes are round-shaped or bean-shaped encapsulated organs.
• They act as filters of lymph to trap antigens.
• They contain two important regions: the outer cortex and inner medulla.
• The cortex has germinal centers with B lymphocytes, surrounded by capsules.
• T cells and dendritic cells are mainly found in the paracortex.

Spleen

• The spleen is a large encapsulated organ found in almost all vertebrate animals.
• It is structured by a network of connective tissue trabeculae.
• The spleen filters blood for blood-borne antigens, cellular debris, and aged blood cells.
• It stores red blood cells, platelets, and recycles iron.

Peyer's Patches

• Peyer's patches are non-encapsulated lymphoid tissue located in the wall of the small intestine.
• In ruminants, pigs, dogs, and rabbits, they are found in the ileum and are primary lymphoid organs for B cell development.
• They contain densely packed lymphoid follicles with only B lymphocytes.
• They undergo involution.