Immunology Basics: Active vs Passive Immunity

Immune System Overview

• Immune system comprises innate and adaptive immunity.

Innate Immunity

• Non-specific defense mechanism against pathogens.
• Acts quickly to eliminate pathogens.
• Initiates and supports adaptive immunity.
• Collaborates with adaptive responses to enhance pathogen elimination.

Adaptive Immunity

• Specific response tailored to individual pathogens.
• Develops memory to facilitate faster response upon re-exposure.
• Requires pre-activation, taking days to weeks.
• Continuously evolves throughout an individual's life.

Key Components of Innate Immunity

• Exterior Defenses:
  • Epithelial barriers, mucosa, and secretions protect from pathogens.
• Cellular Components:
  • Includes natural killer cells, neutrophils, monocytes/macrophages, eosinophils, basophils, mast cells, and platelets.
• Soluble Mediators:
  • Complement: Protein system inducing inflammation and enhancing phagocytosis.
  • Cytokines: Proteins modifying cellular behavior and communications.

Types of Cytokines

• Chemokines:
  • Small cytokines promoting leukocyte migration; increase vascular permeability and blood flow.
• Interferons:
  • Secreted by virus-infected cells; increase resistance to viral replication.
• Acute Phase Proteins:
  • Pro-inflammatory cytokines affecting metabolism, blood flow, and protein production.

Adaptive Immunity Components

• Humoral Immunity:
  • Mediated by antibodies in the bloodstream.
• Cell-Mediated Immunity:
  • Involves T-cells targeting infected or abnormal cells.

Antigens vs. Immunogens

• Antigens:
  • Molecules that bind to antibodies or T-cell receptors.
• Immunogens:
  • Antigens that elicit an immune response; e.g., adjuvants enhance vaccine responses.

Active vs. Passive Immunity

• Active Immunity:
  • Long-lasting protection post-exposure to antigens; includes natural and artificial immunity (vaccines).
• Passive Immunity:
  • Temporary immunity via transferred antibodies; arises naturally (e.g., breastfeeding) or artificially (e.g., antibody injections).

Antibody Structure and Function

• Produced by B lymphocytes.
• Comprised of binding sites with variable regions for antigen specificity and constant regions for structural support.

Major Immunoglobulin (Ig) Classes

• IgM:
  • First responder in immune reactions; activates complement system.
• IgG:
  • Major antibody for bacteria/viruses; long-lasting, crosses placenta.
• IgA:
  • Predominant in mucosal areas; found in secretions, protects mucosal surfaces.
• IgE:
  • Associated with allergies and parasite responses; binds to mast cells.
• IgD:
  • Functions as an antigen receptor on naive B cells.

Clinical Relevance of Cell-Mediated Immunity

• Critical in defending against intracellular pathogens.
• Involves processes like tissue transplant rejection and some autoimmune diseases.

Process of Cell-Mediated Immunity

• Antigen uptake by macrophages/dendritic cells.
• Antigen presentation to T-cells in lymph nodes.
• T-cells activated upon matching T-cell receptors, leading to clonal expansion.
• Memory T-cells form for accelerated response to subsequent exposures.

Formation of T-Lymphocytes

• Stem cells migrate from bone marrow to the thymus for maturation.
• T-cell receptors develop; cells recognizing self-antigens are eliminated to prevent autoimmune issues.

Formation/Maturation of B-Lymphocytes

• B-cells exit bone marrow expressing IgM and IgD.
• Upon activation by antigens, B-cells proliferate and produce antibodies.
• Eventually differentiates into plasma cells and memory B cells for swift secondary responses.

Phases of the Immune Response

• Recognition: Detecting antigens.
• Amplification: Increasing immune cell response.
• Effector: Active elimination of pathogens.
• Termination: Concluding the immune response.
• Memory: Retaining immunity for quicker future responses.

Immunodeficiencies

• Primary Immunodeficiency:
  • Inherent deficits in T-cells or B-cells; often congenital.
• Secondary Immunodeficiency:
  • Resulting from other diseases hindering immune function.

Iatrogenic Immunodeficiency Sources

• Cytotoxic drugs, corticosteroids, immunosuppressive drugs, radiation therapy, splenectomy.