YIJC [H2] EXT A Infectious Disease (N)(S)(vf)

1 2025 JC2 H2 BIOLOGY LECTURE NOTES EXTENSION TOPIC A: INFECTIOUS DISEASE Learning Outcomes: (a) describe the specific (adaptive) immune system, including active, passive, naturally acquired and artificially acquired immunity, and the non-specific (innate) immune system (b) outline the roles of B lymphocytes, T lymphocytes, antigen-presenting cells and memory cells in specific primary and secondary immune responses (c) explain the relationship of the molecular structure of antibodies to their functions, using immunoglobulin G, IgG, as an example (d) explain how somatic recombination, hyper -mutation and class switching result in millions of different antibody molecules (e) discuss how vaccination can control disease (including the eradication of small pox), limited to vaccination stimulates immunity without causing the disease and vaccination of a high enough proportion of the population can break the disease transmission cycle (f) discuss the benefits and risks of vaccination (g) explain how viruses, including influenza virus and HIV, cause diseases in humans through the disruption of host tissue and functions (including HIV and helper T cells, influenza virus and epithelial cells of the respiratory tract) (h) explain the mode of transmission and infection of bacterial pathogens, using Mycobacterium tuberculosis as an example (i) describe the modes of action of antibiotics, including penicillin, on bacteria References: 1. Peter Wood, Understanding Immunology, 3rd Edition: • Chapters 1 – 10 & 14 2. Campbell & Reece, Biology, 11th Edition: • Chapter : Viruses, Pg. 427 – 439 (or corresponding chapter in previous editions) 3. Janeway, et. al., Immunobiology, 6th Edition: 4. Alberts et. Al., Molecular Biology of the Cell, 4th Edition • Chapter 24: The Adaptive Immune System, Pg. 1363 – 1420 Note: Some of these textbooks are available in our library. You may wish to borrow them to supplement your reading when necessary. H2

2 LECTURE OVERVIEW 1) Introduction 3 1.1 Pathogens 4 1.2 The Immune System 5 2) Immune Response 8 2.1 Innate Immunity (Non-Specific) 2.1.1 First Line of Defence: Barriers to Infection 2.1.2 Second Line of Defence: Cellular Defence 2.2 Antigen Presentation 2.3 Adaptive Immunity (Specific) 2.3.1 Cell-Mediated Immunity 2.3.2 Humoral Immunity 11 12 12 17 19 24 28 3) Antibodies 32 3.1 Structure and Function of Antibodies 32 3.2 Generation of Antibody Diversity 36 4) Immunological Memory 40 5) Human Health and Diseases 45 5.1 Viral Diseases 5.1.1 How Influenza Infections Cause Diseases 5.1.2 How HIV Infections Cause Diseases 5.2 Bacterial Diseases (Tuberculosis) 45 47 48 50 6) Medical Interventions 53 6.1 Antibiotics 6.1.1 Inhibition of Protein Synthesis 6.1.2 Inhibition of Nucleic Acid Synthesis 6.1.3 Inhibition of Cell Wall Synthesis 53 54 55 55 6.2 Vaccination 6.2.1 Immune Response 6.2.2 Benefits and Risks of Vaccination 58 64 65 Annex: Glossary of Terms 69

3 1 INTRODUCTION • Immunity is defined as resistance to disease, specifically infectious disease. A disease is termed as infectious when it is caused by a pathogen and is able to be transmitted from one host to another. • The collection of cells, tissues, and molecules that mediate resistance to infections is called the immune system. The function of the immune system is to prevent infections and to eradicate established infections. The coordinated reaction of the cells and molecules in the immune system to infectious microbes is the immune response. • Immunology refers to the study of the immune system, including its responses to microbial pathogens and damaged tissues and its role in disease. PRACTICES OF SCIENCE (POS) POS 1: Understanding the Nature of Scientific Knowledge The origin of immunology is usually attributed to Edward Jenner. He discovered in 1796 that cowpox, or vaccinia, induced protection against human smallpox, an often fatal disease. Jenner called his procedure vaccination, and this term is still used to describe the inoculation of healthy individuals with weakened or attenuated strains of disease-causing agents to provide protection from disease. During the late 19th century, Robert Koch proved that infectious diseases are caused by microorganisms, each one responsible for a particular disease, or pathology. We now recognize four broad categories of disease-causing microorganisms, or pathogens: viruses, bacteria, pathogenic fungi, and parasites. In the 1880s, Louis Pasteur devised a vaccine against cholera in chickens, and developed a rabies vaccine that proved a spectacular success upon its first trial in a boy bitten by a rabid dog. These practical triumphs led to a search for the mechanism of protection and to the development of the science of immunology. In 1890, Emil von Behring and Shibasaburo Kitasato discovered that the serum of vaccinated individuals contained substances —which they called antibodies —that specifically bound to the relevant pathogen. Robert Koch Edward Jenner Louis Pasteur Emil von Behring & Shibasahuro Kitasato 1796 19th C 1880s 1890

4 1.1 PATHOGENS • The types of pathogens that can cause disease include many single-celled microorganisms e.g. viruses, bacteria, yeasts and larger multicellular parasites e.g. fungi and parasitic worms. Fig 1.1.1: Types of pathogens and diseases caused • Pathogens have many different antigens that can be recognised by cells of the immune system. – Antigens are substances that induce an immune response as they are recognised by antibodies and cells of the immune system. Antigens are usually foreign proteins, but can also be carbohydrates, lipids or nucleic acids. – For example, a bacterium’s antigens can be found on its cell wall and flagellum, while a virus’s antigens can be found on its viral glycoprotein. • Cells of the immune system and antibodies bind to a specific part of the antigen known as the epitope. For a protein antigen, the epitope is usually between 8 to 22 amino acids long. Fig.1.1.2: Antigenic epitopes

5 1.2 THE IMMUNE SYSTEM • The immune system functions to remove or neutralise the threats posed to the body by pathogens. • The organs of the immune system are divided into the primary and secondary lymphoid organs. – The two primary lymphoid organs (in most mammals) are the bone marrow and thymus, where naïve lymphocytes develop and maturation occurs – in bone marrow for B cells and thymus for T cells. – Mature lymphocytes will then migrate to the secondary lymphoid organs and tissue which are the lymph nodes, spleen and mucosa surfaces. • The lymphatic vessels connect most tissues in the body with lymph nodes and eventually the bloodstream. The fluid in the lymphatic vessels is known as lymph and is derived from the interstitial fluid surrounding cells in any tissue or organ. • Lymph contains lymphocytes and tissue-derived dendritic cells and other white blood cells e.g. neutrophils, basophils etc, but not red blood cells. A number of these lymphatic vessels meet at specialized structures known as the lymph nodes. Fig. 1.2.1: Lymphoid organs

6 Fig. 1.2.2: Lymphocyte development RECALL Differentiation of Haematopoietic Stem Cells

7 2 IMMUNE RESPONSE • Innate immunity involves the immediate response to an infection. It includes a variety of protective measures that are continually functioning, and provides a first line of defence against pathogens. However, it does not have a memory and is non-specific. • Adaptive immunity develops more slowly, but provides a specific response and long lasting protection against the same pathogen. – Adaptive immunity can be induced by exposure to a foreign antigen resulting in the production of antibodies (i.e. active immunity ), or acqu