ch13

Host Defenses II – Adaptive Defenses & Immunization

Instructor Information

• Instructor: Dr. Steve Bauer, Ph.D.
• Institution: Monroe Community College
• Course: BIO 202-105, 106, & 107 – Spring 2026
• Date: March 19, 2026
• Resource: https://cellcartoons.net/lymphocytes/

Overview of Lecture

• Review of the immune system as a whole
• B & T lymphocytes
• BCR (B Cell Receptor)/Antibody structure
• Antigen recognition by B cells
• TCR (T Cell Receptor) structure
• Antigen recognition by T cells
• Types of T cells:
  • Helper T cells
  • Cytotoxic T cells
  • Regulatory T cells
• Antigen presentation by B cells & innate immune phagocytic cells to T cells

Immune System Overview

Pathogens

• Types: Bacteria, fungi, and viruses

Innate Immunity

• Definition: Recognition of traits shared by a broad range of pathogens using a small set of receptors
• Response Speed: Rapid

Barrier Defenses

• Components:
  • Skin
  • Mucous membranes
  • Secretions

Internal Defenses

• Components:
  • Phagocytic cells
  • Natural killer (NK) cells
  • Antimicrobial proteins
  • Inflammatory response

Humoral Response (B cells)

• Function: Antibodies defend against infection in body fluids

Cell-mediated Response (T cells)

• Function: Defends against infection by directly killing or modulating the immune response against infected cells

Adaptive (Acquired) Immunity

• Definition: Recognition of traits specific to particular pathogens using a vast array of receptors
• Response Speed: Slower
• Develops when the innate immune system cannot eliminate the pathogen
• Involves a specific response to pathogens and can induce memory cells for future infections

Real World Connection

• Goal of Vaccination: To induce memory responses by introducing parts or whole pathogens into the body (e.g., influenza, MMR, COVID-19)

Lymphocytes

• Definition: Key players in adaptive (acquired) immunity
• Proportion: 20-40% of all leukocytes (WBCs)
• Characteristics:
  • Smallest type of WBC
  • Dark, round nucleus
  • Contains little cytoplasm
  • More information:
  • https://ar.inspiredpencil.com/pictures-2023/lymphocytes-under-microscope
  • http://www.pathologystudent.com/?p=2186

Antigen Receptors

B & T Lymphocytes

• Origins:
  • Both produced in the bone marrow
  • B cells mature in the bone marrow
  • T cells mature in the thymus
• Function: Enter circulation and peripheral lymphoid organs (e.g., spleen and lymph nodes) to survey for invading pathogens/tumor cells

Antigens

• Definition: Substances that can elicit a response from a B or T cell
• Epitopes: Small accessible parts of an antigen that bind to an antigen receptor
• Binding: Recognition occurs through antigen receptor binding, which is specific to each antigen

B Cell Receptors (BCRs)

• Structure: Y-shaped protein molecule with:
  • Two identical heavy chains
  • Two identical light chains
• Chains:
  • Heavy and light chains have constant (C) and variable (V) regions
  • Constant Region: Varies little among B cells
  • Variable Region: Differ greatly; forms antigen-binding site
  • The V regions provide binding specificity

Antigen Recognition by B Cells

• Activation Step: Binding of BCR to antigen marks B cell activation; before this, B cells are termed NAÏVE
• Results in the creation of:
  • Plasma Cells: Secrete soluble forms of BCR called antibodies (Ab) or immunoglobulins (Ig)
  • Memory Cells: Record information for future pathogen encounters
• Defense Mechanism: Antibodies bind to pathogens, marking them for elimination (HUMORAL RESPONSE)

BCR Structure Visualization

• Antigen-binding site:
  • Disulfide bridge
  • Light and heavy chains illustrated
  • Components visualization: plasma membrane, cytoplasm of B cell

Antigen Presentation by B Cells

• Process: Involves internalization, processing, and presenting antigen pieces on MHC II to T cells
• Analogy: Vlad the Impaler’s Forest of Corpses

T Cell Antigen Recognition

T Cell Receptor (TCR) Structure

• Composition: Composed of two different polypeptide chains (alpha and beta)
• Regions:
  • V region forms the antigen-binding site
  • Rest is the constant region
• Visual representation of TCR structure

Types of T Cells

Helper T Cells (TH)

• Recognition: Antigen presented via MHC II, express CD4
• Function: Detect pathogens, activating other immune cells
• Subsets:
  • TH1: Primarily intracellular pathogens
  • TH2: Humoral response
  • TH17: Protection at mucosal surfaces

Cytotoxic T Cells (TC/CTL)

• Recognition: Antigen presented via MHC I, express CD8
• Function: Directly destroy virus-infected and tumor cells

Regulatory T Cells (Tregs)

• Recognition: Antigen via MHC II or MHC I, express CD4 or CD8
• Function: Modulate immune response, limit chronic inflammatory diseases

Activation and Antigen Presentation

Antigen Presentation Mechanism

• TCR binds to antigen fragments presented by another cell using MHC molecules
• Types of MHC Molecules:
  • MHC I: Present intracellular antigens
  • MHC II: Present extracellular antigens
• Cells Serving as APCs: B cells, macrophages, and dendritic cells

Immunization: Historical Context

Early Methods

• 6th Century China: Drying and grinding smallpox scabs, blowing into nostrils
• 10th Century Variolation: Inoculation with dried pus from smallpox pustules

Edward Jenner’s Contribution (1796)

• Utilized cowpox to prevent smallpox
• Test on a young boy demonstrated immunity to smallpox post cowpox injection
• Vaccination defined as immunity obtained via inoculation with selected antigens

Types of Immunization

Passive Immunization

• Definition: Treating patients with antibodies for immediate effect without inducing memory
• Use: Especially effective for patients with diminished immune response

Active Immunization (Vaccination)

• Principles: Stimulate primary and memory responses against pathogens
• Effect: Prepares immune system for future pathogen exposure
• Impact: Reduced prevalence of infectious diseases

Active vs Passive Immunity

Types of Vaccines

Whole Organism Vaccines

• Example: Injected flu vaccine
• Example: Measles, mumps, rubella (MMR) vaccine

Antigenic Component Vaccines

• Examples:
  • Hepatitis B vaccine
  • Pfizer & Moderna COVID-19 vaccines
  • Haemophilus influenzae type b conjugated vaccine
  • Johnson & Johnson COVID-19 vaccine

mRNA Vaccine Technology

• Components:
  • Lab-manufactured mRNA coding for SARS-CoV-2 spike protein
  • Lipid vesicles deliver mRNA to human cells
• Outcome: T and B cells become sensitized to the spike protein, preparing for actual virus exposure

Helpful Resources

• Practice Test: Link to Test
• Video Reviews: 2 recommended videos for review included in lecture