RC

Immunity PowerNotes 2021

Immune System Overview

  • Protection relies on two intertwined concepts: Inflammation + Immunity → working together = immunocompetence
  • Immunity = body’s ability to resist disease
  • Three overarching purposes of every immune response
    • Defense
    • Homeostasis (maintaining internal equilibrium)
    • Surveillance (constant scanning for “non-self”)
  • Major anatomical/functional components repeatedly referenced
    • Complement system, thymus, bone marrow, spleen, lymph nodes, tonsils, Peyer’s patches, appendix, skin-associated lymphoid tissue, gut/genital/bronchial lymphoid tissue, hormones that influence immunity
    • Circulating cellular elements = Leukocytes (white blood cells)
  • Each person’s cells display unique surface proteins = Human Leukocyte Antigen (HLA) → think of it as the body’s barcode; only an identical twin shares the same HLA set

Self/Non-Self Recognition & Self-Tolerance

  • Self-tolerance = immune system normally recognizes & respects host tissues
  • Failure → autoimmune disease
  • Immune system must discriminate against a vast range of potential invaders
    • Pathogens: viruses, bacteria, fungi, protozoa
    • Environmental triggers: pollen, bee venom, foods, poison ivy resin
    • Iatrogenic/medical: transplanted tissue & transfused blood
  • When triggered, the system acts to
    1. Neutralize
    2. Destroy
    3. Eliminate

The Three Processes Required for Protection

  1. Inflammation (Innate, non-specific) – rapid mobilization of neutrophils & monocytes
  2. Antibody-Mediated Immunity (AMI) / Humoral Immunity – B-cell driven, immunoglobulin production
  3. Cell-Mediated Immunity (CMI) – T-cell and NK-cell driven, direct cellular attack

Types of Immune Response

  • Innate (Natural) Immunity
    • Immediate, non-specific, present at birth
    • Barriers & reactions
    • Physical: intact skin, mucous membranes
    • Mechanical: coughing, sneezing, vomiting
    • Biochemical: gastric juice, tears, saliva, normal flora, interferons
    • Cellular/chemical: phagocytosis, complement cascade
    • Functions as the first line of defense
  • Adaptive (Acquired) Immunity
    • Specific, has memory → second & third lines of defense
    • Two branches
    1. Antibody-Mediated (B-cell)
    2. Cell-Mediated (T-cell)
    • Acquisition routes
    • Naturally acquired active: infection with pathogen (e.g., \text{COVID-19})
    • Artificially acquired active: vaccination (e.g., mRNA COVID vaccine)
    • Naturally acquired passive: maternal IgG crossing placenta / breast-milk IgA
    • Artificially acquired passive: monoclonal antibody infusion or antiserum (e.g., laboratory-prepared COVID antibodies)

Lymphoid Organs

  • Primary (site of development):
    • Bone marrow – B-cell maturation & all blood cell production
    • Thymus – T-cell maturation, thymic hormones guide differentiation
  • Secondary (site of function/interaction):
    • Lymph nodes, spleen, tonsils, Peyer’s patches, appendix, mucosa-associated lymphoid tissue (MALT) in gut, genital, bronchial, skin

Leukocytes – “The Army”

  • Granulocytes (contain cytoplasmic granules that release histamine, bradykinin, prostaglandins)
    • Basophils – “baaaad” allergies; elevation → allergic / hypersensitive reaction
    • Eosinophils – “eeeeewwww” worms; rise in parasitic infection & allergy
    • Neutrophils – primary bacterial defenders
    • Immature forms = bands (“left shift”) → lots of bands = severe infection/sepsis
    • Mature forms = segs (segmentals)
  • Monocytes / Macrophages – “Pac-Man” professional phagocytes, antigen-presenting cells
  • Lymphocytes
    • B cells → produce antibodies (AMI)
    • T cells → orchestrate & execute CMI (CD4 helpers, CD8 cytotoxic)
    • Natural Killer (NK) cells → innate, nonspecific cytotoxicity against tumor & virus-infected cells

Antibody-Mediated Immunity (Humoral Response)

  • B-cells recognize extracellular antigens → differentiate into plasma cells → secrete immunoglobulins (antibodies)
  • Five major classes (Table relationships captured below)
Immunoglobulin% of total serumMain Location(s)Key Functions
IgG76\%Plasma & interstitial fluid; crosses placentaProvides baseline immunity to newborn; predominant secondary response
IgA15\%Breast milk, tears, saliva; mucosal secretions of respiratory, GI, GU tractsPrevents pathogen adherence/penetration at mucosal surfaces
IgM8\%PlasmaFirst Ig produced on initial exposure (primary response)
IgE0.002\%Plasma & interstitial fluidTriggers Type I rapid hypersensitivity (allergy, anaphylaxis); defense vs. parasites
IgD1\%Plasma (B-cell surface)Plays role in B-cell activation/maturation
  • Memory B-cells persist → quicker, stronger response on re-exposure

Cell-Mediated Immunity (T-cell Dominant)

  • Activated when antigens reside inside cells (viruses, mycobacteria), with fungal infections, transplant rejection, or certain hypersensitivity reactions
  • Major effector cells
    • T-Cytotoxic (CD8) – directly lyse target cells displaying foreign antigen
    • T-Helper (CD4) – “coordinators,” secrete cytokines that amplify B & T responses
    • Clinical tie-in: in HIV, monitor CD4; when count drops below 200 cells/µL → AIDS → high susceptibility to opportunistic infections
    • Natural Killer (NK) – innate lymphocytes, no prior sensitization, attack tumor & virus-infected cells; bridge innate + adaptive
  • Visual mnemonic: killer T cell saying “You’re history!” while terminating cancer/virus-infected cell
  • Group of >30 plasma proteins that, once activated, opsonize pathogens, recruit inflammatory cells, and puncture membranes (MAC formation)
  • Tightly regulated to avoid host damage

Categories of Immunity – Summary Matrix

  • Active vs. Passive & Natural vs. Artificial
Active (own antibodies)Passive (ready-made antibodies)
NaturalInfection (e.g., varicella)Maternal IgG/IgA
ArtificialVaccination (e.g., COVID shot)Antiserum / monoclonal antibodies (e.g., lab-made COVID Ig)
  • Active = long-lasting (memory)
  • Passive = immediate but temporary (no memory)

Inflammatory Response – Key Points (Innate)

  • Vasodilation, ↑ vascular permeability, leukocyte migration
  • Chemical mediators released by granulocyte granules: histamine, bradykinin, prostaglandins
  • Cardinal signs: redness, heat, swelling, pain, loss of function

Mechanical/Physical/Chemical Barriers – First Line Specifics

  • Intact skin & mucous membranes
  • Secretions containing lysozyme (tears, saliva)
  • Gastric acidity, normal flora competition
  • Mechanical clearance: cilia movement, coughing, sneezing, vomiting, urine flow
  • Interferons – antiviral cytokines produced by infected cells
  • Transplantation: HLA matching crucial to minimize rejection (CMI involvement)
  • Hypersensitivity reactions: Elevation in basophils/eosinophils & IgE hallmark; clinical preparedness for anaphylaxis
  • Vaccination: Harnesses adaptive active immunity without disease burden; central to public health
  • Monoclonal antibody therapy: Example of passive artificial immunity, offers targeted immediate protection (e.g., COVID neutralizing Abs)
  • Autoimmunity: Breakdown of self-tolerance; understanding antigen recognition drives therapeutic development

Quick Memory Hooks & Metaphors

  • Basophils = “Baaaad allergies”
  • Eosinophils = “Eeeew worms”
  • Macrophages = “Pac-Man” gobbling pathogens
  • Bands vs. Segs = baby vs. adult neutrophils; lots of babies → severe infection
  • HLA = body’s barcode

Numerical / Statistical References Recap

  • Immunoglobulin serum percentages listed in LaTeX above (e.g., 76\% IgG)
  • CD4 threshold for AIDS diagnosis: <200 cells/µL
  • Complement cascade involves >30 proteins

Foundational Connections to Previous Lectures (Implied)

  • Inflammation mediators parallel clotting cascade topics
  • Relationship between endocrine hormones (e.g., cortisol) and immune suppression

Study Strategy Tips

  • Master distinctions: Innate vs. Adaptive; AMI vs. CMI
  • Associate immunoglobulin class with location & role (e.g., IgA → mucosa)
  • Remember cellular nicknames to cue test answers (Pac-Man, worms, allergies)
  • Use the active/passive matrix to categorize clinical scenarios quickly
  • Practice interpreting CBC w/ differential: look for bands, eosinophilia, basophilia as clues