Adaptive (Acquired) Immunity – B & T Lymphocytes, Humoral vs. Cell-Mediated

Session Objectives

• Recognize all components of the acquired (adaptive) immune response.
• Describe the two key lymphocyte populations—B lymphocytes (B cells) and T lymphocytes (T cells)—including nomenclature swaps (e.g., “B lymphocyte/B cell”).
• Explain how B cells generate the humoral immune response (production of antibodies = immunoglobulins).
• Explain how T cells generate the cell-mediated immune response.
• Define and differentiate antigens, haptens, epitopes (teased; full definitions promised in upcoming segment).
• Outline the role of lymphoid organs (thymus, lymph nodes) and accessory cells (macrophages, dendritic cells) in adaptive immunity.
• Illustrate how immunization exploits these principles.

Dual Nature of Acquired Immunity

• Acquired immunity splits into two inter-dependent arms (an artificial division used for teaching):
  • Humoral immunity
  • Mediated by B cells → plasma cells → antibodies (immunoglobulins).
  • Effective only outside cells (extracellular microbes, circulating toxins, free viral particles).
  • Cell-mediated immunity
  • Mediated by T cells (cytotoxic, helper, others).
  • Targets infected, altered, or foreign host cells (virus-infected cells, intracellular bacteria, fungi, eukaryotic parasites, cancer cells).
• The two arms cooperate; neither works effectively in isolation.

Historical Milestones

• 1890 – Kitasato & von Behring
  • Observed that animals surviving infection possessed a transferable protective factor in their serum (straw-coloured, cell-free blood fraction).
  • Demonstrated passive immunity: serum from immune animal → naive animal → protection.
  • Implied the factor could:
  • Recognize foreign material.
  • Distinguish self from non-self.
  • Operate with high specificity.
• 1960s – Walter & Eliza Hall Institute (Melbourne)
  • Discovered serum antibodies were insufficient against some diseases unless white blood cells were co-transferred.
  • Led to recognition of two distinct effectors: antibody-mediated and cell-mediated mechanisms.
  • Identified B lymphocytes (antibody producers) and T lymphocytes (cellular effectors).

Core Characteristics of Acquired Immunity

• Specificity – Targets a unique antigenic determinant on each pathogen.
• Memory – Second exposure → faster (≈ $1{-}2$ days) and stronger response vs. primary exposure (≈ $\ge 10$ days to peak).
• Recognition of Self vs. Non-Self – Avoids attacking own tissues (tolerance).
• Amplification of Inflammation & Complement Activation – Adaptive products enhance innate processes.

Humoral Immunity: B Cells & Antibodies

• Terminology origin: “Humoral” derives from Greek concept of body “humors” (fluids).
• B lymphocytes
  • Circulate through blood, lymph, and secondary lymphoid tissues.
  • Upon antigen encounter, differentiate → plasma cells secreting antibodies (immunoglobulins).
• Antibody functions (high-level preview)
  • Bind extracellular bacteria, toxins (e.g., tetanus toxin), and free viruses.
  • Unable to enter host cells ⇒ ineffective once pathogen becomes intracellular.
• Viral limitation
  • Work only before viral entry; once virus integrates into a host cell, humoral immunity alone cannot clear it.

Cell-Mediated Immunity: T Cells

• T lymphocytes
  • Recognize and destroy infected or abnormal host cells.
  • Methods:
  • Direct cytotoxic killing of infected cells.
  • Secretion of cytokines → intensify inflammation; recruit other immune cells.
• Essential for control of:
  • Intracellular viruses.
  • Intracellular bacteria (e.g., Mycobacterium spp.).
  • Intracellular parasites (e.g., Plasmodium in malaria).
  • Fungi and eukaryotic parasites.
  • Cancer surveillance and elimination.

Clinical & Pathological Correlates

• Success = Immunity (protection without disease).
• Failure scenarios
  • AIDS (HIV targets CD4⁺ helper T cells) → collapse of both cell-mediated and humoral arms.
  • Cancer – Tumour cells evade or suppress immune surveillance.
  • Recurrent / life-threatening infections – seen with genetic immunodeficiencies, chemotherapy, or immunosuppressive drugs.
• Studying failures (AIDS, cancer, severe infections) illuminates how each immune component normally works.

Key Cell Types & Their Functions (Snapshot)

(“•” represents the image placeholders described in the transcript.)

Immunodeficiency Examples & Student Q&A Highlights

• HIV infection – Profound loss of helper T cells; suppresses both B- and T-cell functions.
• Primary (genetic) immunodeficiencies – Rare defects leading to absent B-cell, T-cell, or combined functions.
• Chemotherapy – Targets rapidly dividing cells; collateral damage to lymphocyte precursors → transient immunosuppression.
• Classroom dialogue clarified that insufficient B/T production can stem from infection (HIV), genetics, or drugs.

Pending Topics (Preview)

• Detailed definitions of antigens, haptens, and epitopes.
• Mechanistic explanation of immunization/vaccination strategies.
• Step-wise pathways for antibody production, T-cell activation, and their molecular signaling.
• Expanded discussion of the complement system interaction with adaptive immunity.

Cheat-Sheet Summary

• Adaptive immunity is specific, has memory, discriminates self/non-self, and amplifies innate defenses.
• Humoral arm (B cells → antibodies) handles extracellular threats.
• Cell-mediated arm (T cells) handles intracellular threats and abnormal host cells.
• Clinical observations (serum transfer, white-cell transfer) provided the historical basis for this dual-arm model.
• Failures of either arm (HIV, genetic defects, chemotherapy) manifest as increased infections and/or cancers.
• Understanding each leukocyte’s role helps interpret clinical blood counts and immunopathology.