Preview
IB DP Biology — C: Interaction and Interdependence | SL | C3.2 Defence Against Disease
📋 What You Will Learn
- Identify the physical and chemical barriers that form the body's first line of defence against pathogens
- Explain the cellular mechanisms of the innate immune response, including phagocytosis and the inflammatory response
- Describe how the adaptive immune system mounts a specific response through B lymphocytes and T lymphocytes
- Explain the molecular basis of antibody production, clonal selection, and immunological memory
- Analyse data from immunological experiments, including antibody titre graphs and vaccination studies
- Apply understanding of immunity to evaluate the biological basis of vaccination and autoimmune disease
🔍 Guiding Question How does the immune system distinguish between self and non-self, and how does this distinction allow the body to mount both immediate and long-lasting defences against disease?
🌍 Why This Matters When the COVID-19 pandemic swept the world in 2020, the race to develop vaccines depended entirely on understanding how the adaptive immune system learns to recognise a pathogen and remember it. Every vaccine ever made — from smallpox to influenza to mRNA vaccines — is an engineered conversation with your immune system's memory cells.
The Problem the Body Had to Solve
You are under constant biological siege. Every breath draws in thousands of microorganisms. Every surface you touch transfers bacteria and viruses to your skin. Every meal introduces foreign proteins into your digestive tract. The fundamental challenge of multicellular life is this: how do you remain open to the world — breathing, eating, reproducing — while preventing that openness from killing you?
Evolution's answer is layered, elegant, and ruthlessly efficient. The immune system operates in two distinct but coordinated modes: a fast, non-specific innate response that attacks anything foreign, and a slower, exquisitely specific adaptive response that learns, remembers, and improves with every encounter. Understanding both — and how they communicate — is the heart of this topic.
📖 Key Definition — Pathogen A pathogen is an organism or agent capable of causing disease in a host. Pathogens include bacteria, viruses, fungi, protists, and prions. They cause disease by directly damaging host cells, competing for resources, or releasing toxins.
📖 Key Definition — Antigen An antigen is any molecule — typically a protein or glycoprotein on the surface of a pathogen, toxin, or foreign cell — that is recognised by the immune system and triggers an immune response. The word combines "antibody generator." Antigens are the molecular identity cards that the immune system reads to distinguish self from non-self.
The First Line of Defence: Physical and Chemical Barriers
Before any immune cell is ever activated, the body has already deployed its most ancient defences. These are not immune responses in the classical sense — they are structural and chemical barriers that prevent pathogens from entering the body in the first place. Think of them as the walls and moat of a fortress, designed so the battle never has to begin.
Physical Barriers
The skin is the body's largest organ and its most important physical barrier. The outermost layer — the epidermis — consists of tightly packed, keratinised cells that are continuously shed, removing any microorganisms that have attached to the surface. Intact skin is essentially impermeable to most pathogens.
Mucous membranes line all body cavities that open to the external environment: the respiratory tract, the digestive tract, the urogenital tract, and the conjunctiva of the eye. These membranes secrete mucus — a viscous glycoprotein gel — that traps pathogens and particles before they can reach underlying tissues. In the respiratory tract, cilia on epithelial cells beat rhythmically to sweep mucus (and its trapped cargo) upward toward the throat, where it is swallowed and destroyed by stomach acid. This is the mucociliary escalator.
Chemical Barriers
Physical barriers are reinforced by chemical weapons. Lysozyme is an enzyme found in