Cells of the Immune System

Overview

Topic: Cells of the Immune System in veterinary immunology; focus on hematopoiesis, immune cell lineages, innate vs adaptive cells, lymphoid organs, lymphocyte maturation and recirculation, and a clinical case (Canine Trapped Neutrophil Syndrome).
Learning objectives reflected in transcript: differences between primary and secondary lymphoid organs; hematopoiesis; immunologic characteristics of lymphocytes, phagocytes, dendritic cells; microanatomy of lymph nodes; lymphocyte recirculation pathway; reference texts cited (Day’s Veterinary Immunology; Tizard).

Definition: Production of immune cells, red blood cells, platelets from bone marrow.
Scale: approximately 10^{11} to 10^{12} cells per day.
Key cell in bone marrow: hematopoietic stem cell; from this, multipotent and committed hematopoietic progenitors arise, which differentiate into blood cells that enter circulation.

Three major lineages:
- Erythroid lineage: erythrocytes and platelets
- Myeloid lineage: granulocytes (neutrophils, eosinophils, basophils), mast cells, monocytes, macrophages, myeloid dendritic cells
- Lymphoid lineage: lymphocytes (T and B cells)

Colony-stimulating factors (CSFs):
- GM-CSF (granulocyte-monocyte CSF)
- G-CSF (granulocyte CSF)
Interleukins (IL): IL-1, IL-3, IL-4, IL-5, IL-6, IL-7
Role of growth factors: regulate expression of specific receptors, production of effector molecules, and morphological maturation of cells toward their final forms.

Eosinophils: driven by IL-5, IL-3, GM-CSF
Neutrophils: driven by G-CSF, GM-CSF, IL-3
Monocytes: driven by M-CSF, GM-CSF, IL-3
Basophils and mast cells: driven by IL-4, IL-9, GM-CSF
Overall: growth factors influence differentiation from progenitors at bone marrow, with cells entering blood and then tissues as they mature.

Pathways flow from: Hematopoietic Stem Cell (bone marrow) → Common Myeloid Progenitor OR Common Lymphoid Progenitor
Myeloid branch gives rise to: NK/T cell precursors, neutrophils, eosinophils, basophils, monocytes, macrophages, dendritic cells, mast cells, platelets, erythrocytes
Lymphoid branch gives rise to: NK cells, B cells, T cells
NK cells and some progenitors have overlapping or alternative routes; mature cells populate blood and tissues; effector functions executed in various organs (e.g., lymphoid tissues, mucosa, and peripheral tissues).