The Immune System: Key Organs, Cells, and Molecules

The immune system is a distributed network of organs, cells, and molecules defending against pathogens and maintaining homeostasis.

Primary Lymphoid Organs (Lymphocyte Development)

• Bone Marrow: Hematopoiesis → all blood cells. B-cell maturation (mammals).
• Thymus: T-cell maturation, selection (positive/negative), central tolerance.

Secondary Lymphoid Organs (Immune Activation)

• Lymph Nodes: Filter lymph; B-cell follicles, T-cell zones; antigen presentation.
• Spleen: Filters blood; red pulp (RBC clearance), white pulp (immune response to blood-borne antigens).
• MALT (Mucosa-Associated Lymphoid Tissue):
  • GALT: Peyer's patches (ileum), appendix.
  • BALT: Tonsils (palatine, tubal, lingual, pharyngeal/adenoids).

Lymphatic Vasculature

• Blind-ended capillaries → collecting vessels → thoracic duct (left) / right lymphatic duct → venous return.
• Transport lymph (immune cells, antigen, fluid).

Key Cellular Players

Lineage	Cells	Key Functions
Myeloid	Neutrophils	Phagocytosis, NETosis, first responders
	Monocytes/Macrophages	Phagocytosis, antigen presentation, tissue repair
	Dendritic Cells	Professional APCs; bridge innate-adaptive
	Eosinophils	Parasites, allergic inflammation
	Basophils / Mast Cells	Allergy, anaphylaxis, IgE-mediated
Lymphoid	B Cells	Antibody production, memory, antigen presentation
	T Cells	CD4+ Helper (Th1, Th2, Th17, Tfh, Treg); CD8+ Cytotoxic; γδ T, NKT
	NK Cells	Innate cytotoxicity (missing self), ADCC
	ILCs	Innate lymphoid cells; tissue immunity

Critical Soluble Mediators

• Cytokines: IL-1, IL-2, IL-6, IL-12, IL-17, IL-23, TNF-α, IFN-γ, TGF-β, chemokines.
• Antibodies (IgG, IgM, IgA, IgE, IgD): Neutralization, opsonization, complement activation, ADCC.
• Complement: C3b opsonization, C5a chemotaxis, MAC lysis.
• Acute Phase Proteins: CRP, ferritin, hepcidin.

This overview captures core components; immune regulation involves intricate checkpoints, metabolic reprogramming, and tissue-specific adaptations.