Immune System Overview Quiz

Immune System Overview

• Comprises primary and secondary organs
• Primary organs: Thymus and Bone Marrow
• Secondary organs: Spleen, lymph nodes, MALT (mucosa-associated lymphoid tissue)

Primary Immune System Organs

• Thymus:

  • Located in the thorax, bilobed structure, regresses after puberty
  • Divided into cortex (many thymocytes) and medulla (few thymocytes)
  • Contains epithelial cells, dendritic cells, and macrophages
  • Over 95% of cortical lymphocytes undergo negative selection for self-tolerance

• Bone Marrow:

  • Source of hematopoietic cells for lymphocyte precursors
  • Location: "islands" of hematopoietic tissue amidst fat in long and flat bones
  • Maturation of B lymphocytes occurs here

Hematopoiesis

• Self-renewing process from pluripotent stem cells
• Generates various blood and immune cells

Secondary Immune System Organs and Tissues

• Locations for mature lymphocytes to encounter antigens
• Composed of lymph vessels and lymphoid organs

Lymphatic System

• Lymphatic capillaries extract interstitial fluid, forming lymph
• Converge at lymph nodes where antigens and antigen-presenting cells arrive via afferent vessels
• Thoracic duct drains lymph into the superior vena cava

Lymph Nodes

• Functions: antigen contact and T-B cooperation
• Structures:
  • Cortex: B cells (with primary and secondary follicles)
  • Paracortex: T cells
  • Medulla: Activated T and B cells, macrophages

Spleen

• Functions: elimination of old red blood cells and filtration of blood antigens
• Histology:
  • Red pulp rich in macrophages for erythrocyte clearance
  • White pulp functions similarly to lymph nodes with PALS (T lymphocytes) and a marginal zone for B lymphocytes

Mucosal Immunity (MALT)

• Includes Waldeyer’s ring (tonsils, adenoids) and MALT tissues in respiratory, genitourinary, and gastrointestinal tracts
• Peyer patches: Specialized lymphoid tissue in the small intestine
  • Houses T and B lymphocytes, involved in IgA production

Immune Cells

• Neutrophils:

  • 90% of granulocytes, short lifespan (2-3 days)
  • Key function: phagocytosis, increases during bacterial infections

• Eosinophils:

  • 1-3% of leukocytes, defense against helminths
  • Contain bilobulated nucleus and granules that bind acidic dyes

• Basophils:

  • <1% of leukocytes, involved in defending against parasites and allergy reactions
  • Contain granules with histamine, heparin, and serine proteases

• Mast Cells:

  • Derived from bone marrow, found in skin and mucosal epithelium
  • Release cytokines and histamine, linked to allergic responses

• Monocytes:

  • Horseshoe-shaped nucleus, migrate to tissues to become macrophages
  • Functions include phagocytosis and acting as antigen-presenting cells (APCs)

• Dendritic Cells:

  • Major APCs with phagocytic activity
  • Classify into classic DCs (myeloid origin) and follicular DCs (not phagocytic)

Lymphocytes

• Comprise 20-40% of total leukocytes

• B Lymphocytes:

  • Mature in bone marrow and recognize soluble antigens via B cell receptors (BcR)
  • Activate, proliferate into memory B cells and plasma cells producing specific immunoglobulins (IgM, IgG, IgA, IgE)

• T Lymphocytes:

  • Differentiate in the thymus, participate in cell-mediated immunity
  • Require activation by antigens and cytokines from helper T cells for effective immune response

• Natural Killer (NK) Cells:

  • Part of innate immune response, responsible for targeting and destroying infected or cancerous cells

Cytokine Production

• Activated macrophages produce various cytokines (e.g., IL-1, IL-6, TNF-α) for inflammatory responses
• Promote differentiation of T lymphocytes and recruit various leukocyte populations to infection sites
• Anti-inflammatory cytokines (e.g., IL-10, TGF-β) mediate tolerance and inflammatory response downregulation