Chapter 11: Immunity in Humans

Learning Objectives

By the end of this chapter, you should be able to:

• Describe the three lines of defense in the human immune system
• Explain specific and non-specific immune responses
• Understand the roles of different white blood cells
• Compare active and passive immunity
• Analyze vaccination principles and their importance

Overview

The human immune system is a complex network of defenses that protects the body from harmful pathogens, foreign substances, and abnormal cells. It operates through multiple layers of protection, from physical barriers to highly specific immune responses. Understanding immunity is crucial for comprehending how the body fights infections, prevents diseases, and maintains overall health.

Body's Defense System

Three Lines of Defense

The immune system is organized into three overlapping lines of defense that provide comprehensive protection against pathogens.

First Line of Defense: Physical and Chemical Barriers

Non-specific defense that prevents pathogens from entering the body

Physical Barriers:

Chemical Barriers:

Importance:

• Prevents most pathogen entry
• Non-specific - works against various pathogens
• Constant protection without prior exposure

Did You Know? The skin is the body's largest organ and sheds about 30,000-40,000 dead skin cells every minute, creating a physical barrier that is constantly renewed to prevent pathogen entry!

Barrier Efficiency Equation:

$$\text{Protection Effectiveness} = \frac{\text{Pathogens Blocked}}{\text{Total Pathogens Encountered}} \times 100\%$$

Second Line of Defense: Internal Cellular and Chemical Defenses

Non-specific defense that activates when pathogens breach first line

Inflammatory Response:

Process:

1. Tissue Damage: Pathogens or injury damage tissues
2. Chemical Release: Damaged cells release histamine and other chemicals
3. Blood Vessel Changes: Vasodilation increases blood flow
4. Plasma Leakage: Increased permeability allows white blood cells to exit vessels
5. Phagocytosis: White blood cells engulf and digest pathogens

Signs of Inflammation:

• Redness (Rubor): Increased blood flow
• Heat (Calor): Increased blood temperature
• Swelling (Tumor): Fluid leakage into tissues
• Pain (Dolor): Pressure on nerve endings
• Loss of Function (Functio Laesa): Impaired tissue function

Phagocytic Cells:

Phagocytosis Efficiency Equation:

$$\text{Phagocytic Rate} = \frac{\text{Number of Pathogens Engulfed}}{\text{Time} \times \text{Number of Phagocytes}}$$

Fever Response:

• Trigger: Pyrogens (fever-inducing substances) from pathogens
• Mechanism: Hypothalamus raises body temperature set point
• Benefits:
  • Inhibits pathogen growth
  • Enhances immune cell activity
  • Improves antibody production

Natural Killer (NK) Cells:

• Function: Destroy virus-infected and cancerous cells
• Mechanism: Detect abnormal cells and induce apoptosis
• Importance: Early defense against viral infections and cancer

Third Line of Defense: Specific Immune Response

Highly specific defense involving lymphocytes and memory cells

Lymphocytes: The main cells of specific immunity

• T Lymphocytes (T cells): Cell-mediated immunity
• B Lymphocytes (B cells): Humoral immunity
• Origin: Bone marrow, mature in thymus (T cells) and bone marrow (B cells)

Antigen Recognition:

• Antigens: Foreign molecules that trigger immune response
• Epitopes: Specific parts of antigens recognized by antibodies/T cells
• Antibodies: Y-shaped proteins produced by B cells
• Receptors: Specific receptors on lymphocytes recognize antigens

Specific Immune Responses:

T Lymphocytes (T Cells)

Types and Functions:

T Cell Activation Process:

1. Antigen Presentation: Antigen-presenting cells display antigens
2. Co-stimulation: Helper T cells provide activation signals
3. Proliferation: Clonal expansion of specific T cells
4. Differentiation: T cells become effector or memory cells
5. Effector Functions: Destruction of target cells or immune coordination

B Lymphocytes (B Cells)

Functions:

• Antibody Production: Create specific antibodies against antigens
• Antigen Presentation: Similar to macrophages
• Memory Cell Formation: Create long-lasting immunity

Antibody Structure:

• Y-shaped protein with two identical heavy chains and two identical light chains
• Variable regions: Bind to specific antigens
• Constant regions: Determine antibody class and function

Antibody-Antigen Binding Equation:

$$\text{Binding Affinity} = \frac{\text{Bound Antibodies}}{\text{Total Antibodies}}$$

Antibody Specificity:

$$\text{Specificity} = \frac{\text{Correct Antigen Binding}}{\text{Total Binding Events}}$$

Antibody Classes (Immunoglobulins):

Antibody Functions:

• Neutralization: Block pathogen attachment/invasion
• Opsonization: Mark pathogens for phagocytosis
• Complement Activation: Trigger inflammatory response
• Agglutination: Clump pathogens together for easier removal

Antibody-Pathogen Interaction:

$$\text{IgG} + \text{Pathogen} \rightarrow \text{Immune Complex}$$ $$\text{Immune Complex} + \text{Macrophage} \rightarrow \text{Phagocytosis}$$

Types of Immunity

Active Immunity

Definition: Body produces its own antibodies in response to antigen exposure

Characteristics:

• Long-lasting: Provides years or lifetime protection
• Delayed onset: Takes time to develop (1-2 weeks)
• Memory cells: Creates immunological memory

Natural Active Immunity

• Source: Natural infection with pathogen
• Examples: Recovering from chickenpox, measles
• Advantages: Strong, long-lasting protection
• Disadvantages: Risk of disease complications

Artificial Active Immunity

• Source: Vaccination with weakened/attenuated pathogens
• Examples: Measles vaccine, polio vaccine, COVID-19 vaccine
• Mechanism: Simulates infection without disease
• Benefits: Safe, confers immunity, reduces disease spread

Passive Immunity

Definition: Receive antibodies from external source

Characteristics:

• Immediate protection: No waiting period
• Short-term: Lasts weeks to months
• No memory cells: No immunological memory

Natural Passive Immunity

• Source: Transfer from mother to fetus/infant
• Mechanisms:
  • Placenta: IgG antibodies cross placenta during pregnancy
  • Breast milk: IgA antibodies provided through breastfeeding
• Benefits: Protects newborns before their immune system matures

Artificial Passive Immunity

• Source: Injection of pre-formed antibodies
• Examples:
  • Antiserum: Antibodies against specific toxins (snake venom, tetanus)
  • Immune Globulin: pooled antibodies for specific diseases
• Uses: Post-exposure prophylaxis, treatment for certain infections
• Duration: Temporary protection (weeks to months)

Comparison of Immunity Types

Vaccination and Immunization

Principles of Vaccination

Herd Immunity: When enough people are vaccinated, pathogens cannot spread effectively, protecting even unvaccinated individuals

Types of Vaccines:

Vaccination Schedule:

• Infancy: Multiple vaccines for preventable diseases
• Childhood: Boosters and additional vaccines
• Adolescence: Tdap, HPV, meningococcal vaccines
• Adulthood: Annual flu shots, tetanus boosters, travel vaccines

Vaccine Development and Testing

Clinical Trials Process:

1. Preclinical Testing: Laboratory and animal studies
2. Phase I: Small human trials (20-100 people)
3. Phase II: Larger human trials (100-1000 people)
4. Phase III: Large-scale trials (thousands of people)
5. Phase IV: Post-marketing surveillance

Safety and Efficacy:

• Safety: Monitor for adverse reactions
• Efficacy: Measure effectiveness in preventing disease
• Regulatory Approval: Agencies like FDA, EMA evaluate data

Vaccine Efficacy Calculation:

$$\text{Efficacy} = \frac{\text{Attack Rate}_{control} - \text{Attack Rate}_{vaccinated}}{\text{Attack Rate}_{control}} \times 100\%$$

Herd Immunity Threshold:

$$\text{Threshold} = 1 - \frac{1}{R_0}$$

where $R_0$ = basic reproduction number

Immune Disorders

Hypersensitivity Reactions

Overactive immune responses to harmless substances

Autoimmune Diseases

Immune system attacks body's own tissues

Immunodeficiency Disorders

Reduced immune function leading to increased infection risk

Primary Immunodeficiencies (Genetic)

• Severe Combined Immunodeficiency (SCID): Absent T and B cells
• Common Variable Immunodeficiency (CVID): Low antibody production
• DiGeorge Syndrome: Thymus development defect

Secondary Immunodeficiencies (Acquired)

• HIV/AIDS: CD4+ T cell destruction
• Malnutrition: Impaired immune cell function
• Chemotherapy: Bone marrow suppression
• Aging: Immunosenescence, reduced immune function

Laboratory Investigation of Immunity

Blood Tests for Immune Function

Complete Blood Count (CBC):

• White Blood Cell Differential: Neutrophils, lymphocytes, monocytes, eosinophils, basophils
• Abnormalities: Infections, allergies, immunodeficiencies

Specific Immune Tests:

• Antibody Titers: Measure antibody levels against specific antigens
• T Cell Subset Analysis: CD4/CD8 ratios in HIV monitoring
• Complement Levels: Assess complement system function
• Autoantibody Tests: Detect antibodies against self-antigens

Allergy Testing:

• Skin Prick Tests: Localized reaction to allergens
• Blood Tests: IgE levels against specific allergens
• Elimination Diets: Identify food allergies

Immunological Techniques

Microscopy Methods:

• Light microscopy: Visualize immune cells in blood smears
• Fluorescence microscopy: Detect specific antigens with fluorescent antibodies
• Electron microscopy: Detailed examination of immune cell structures

Molecular Methods:

• PCR: Detect pathogen DNA/RNA
• ELISA: Quantify antigens and antibodies
• Flow Cytometry: Analyze immune cell populations
• Western Blotting: Identify specific proteins

Practice Tips for SPM Students

Key Concepts to Master

1. Three lines of defense and their specific mechanisms
2. Types of immunity (active vs. passive, natural vs. artificial)
3. Lymphocyte functions and antigen recognition
4. Antibody structure and classes
5. Vaccination principles and herd immunity

Experimental Skills

1. Identify immune cells from blood smears and diagrams
2. Design immunity experiments with proper controls
3. Interpret immune test results and clinical significance
4. Apply knowledge to vaccination and disease prevention scenarios

Problem-Solving Strategies

1. Immune response analysis: Understand the sequence of defense activation
2. Vaccination planning: Calculate optimal vaccination schedules
3. Disease outbreak analysis: Use herd immunity concepts
4. Clinical case studies: Apply immune knowledge to patient scenarios

Environmental and Health Connections

Environmental Impact on Immunity

• Pollution can impair immune function and increase allergy risk
• Climate change affects disease patterns and immune adaptation
• Microbiome diversity influenced by environment and diet
• Stress affects immune system regulation

Public Health Significance

• Vaccination programs prevent millions of deaths annually
• Antibiotic resistance challenges immune system effectiveness
• Pandemic preparedness relies on immune system understanding
• Healthcare worker protection uses immune knowledge for infection control

Biomedical Applications

• Monoclonal antibodies for cancer and autoimmune diseases
• Immunotherapy for cancer treatment
• Stem cell transplantation for immunodeficiencies
• Diagnostic testing for immune function assessment

Summary

• The immune system has three lines of defense: physical barriers, internal cellular defenses, and specific immune responses
• Non-specific immunity provides rapid, broad protection against various pathogens
• Specific immunity involves lymphocytes, antibodies, and immunological memory
• Different types of immunity (active, passive, natural, artificial) provide varying levels and durations of protection
• Vaccination is a safe and effective way to achieve active artificial immunity
• Understanding immunity is crucial for preventing and treating diseases

Related Topics

• Chapter 10: Transport in Humans and Animals
• Chapter 12: Coordination and Response in Humans
• Chapter 13: Homeostasis and the Human Urinary System