Responding to Antigens
The immune system provides a multi-layered defence against pathogens. This guide explores the progression from initial, non-specific barriers to the powerful, specific, and long-lasting adaptive immune response that protects us from disease.
The Three Lines of Defence
1st Line: Barriers to Entry
Physical (skin), chemical (stomach acid), and microbiological (normal flora) barriers that prevent pathogens from entering the body.
2nd Line: Innate Immune Response
A rapid, non-specific response if the first line is breached. Involves inflammation and phagocytic cells like macrophages.
3rd Line: Adaptive Immune Response
A highly specific response targeting particular pathogens. Involves B and T lymphocytes, antibody production, and immunological memory.
First Line of Defence: Barriers
The most effective defence is preventing pathogen entry. The first line consists of robust physical, chemical, and microbiological barriers at the body’s surfaces. Explore the different types of barriers in animals and plants below.
- Physical Barriers: Intact skin with its tough keratin layer forms a formidable shield. On internal surfaces, mucous membranes trap pathogens, and cilia (in the respiratory tract) sweep them out via the mucociliary escalator.
- Chemical Barriers: Acidic environments on the skin (sebum, sweat) and in the stomach are hostile to microbes. Lysozyme, an enzyme in tears and saliva, degrades bacterial cell walls.
- Microbiological Barriers: The body’s “normal flora” (commensal bacteria) outcompetes pathogenic invaders for resources and attachment sites, a process known as competitive exclusion.
- Physical Barriers: Plants possess a waxy cuticle on leaves, thick bark on stems, and thorns to deter herbivores. They can also form galls to isolate infections.
- Chemical Barriers: Plants produce a vast arsenal of antimicrobial compounds like saponins (disrupt cell membranes) and tannins (inactivate enzymes) that are released upon damage to repel or kill pathogens.
The Innate Response
The second line of defence launches a rapid, non-specific counter-attack. It involves cells, chemical signals, and the crucial process of inflammation to contain any pathogen that breaches the initial barriers.
Cellular Operatives
Neutrophil
Macrophage
Dendritic Cell
Mast Cell
NK Cell
Complement
The Inflammatory Response
The Adaptive Response
The third line of defence is specific, adaptive, and creates memory. It is coordinated in the lymphatic system and carried out by two interconnected branches: humoral and cell-mediated immunity.
The lymphatic system is the command and control network for the adaptive response. It transports immune cells and filters lymph fluid for pathogens.
- Primary Lymphoid Organs: Where lymphocytes are made and mature.
- Bone Marrow: Site of production for B and T cells; B cells also mature here.
- Thymus: Site where T cells migrate to mature and become self-tolerant.
- Secondary Lymphoid Organs: Where mature lymphocytes are activated.
- Lymph Nodes: Filter lymph and are the primary site where APCs activate lymphocytes.
- Spleen: Filters blood and initiates immune responses to blood-borne pathogens.
Activation Pathway: Humoral Response
Follow the steps to see how B cells are activated to produce antibodies against extracellular threats.
Activation Pathway: Cell-Mediated Response
Follow the steps to see how Cytotoxic T cells are activated to destroy compromised host cells.
Acquiring Immunity & Memory
Immunity can be acquired naturally or artificially, and results in either active production of memory cells or passive, temporary protection. Explore the four types, then see how memory works in the chart below.
Four Types of Acquired Immunity
Natural Active
Artificial Active
Natural Passive
Artificial Passive
Primary vs. Secondary Response
Disease Challenges & Control
Controlling the spread of infectious disease requires a combination of scientific strategies and social cooperation. Explore the case studies below to understand these dynamics.
Herd immunity provides indirect protection to vulnerable individuals when a high percentage of the population is immune, typically through vaccination. This breaks the chains of transmission, making it difficult for a pathogen to spread.
- Benefit: Protects infants, the elderly, and immunocompromised individuals who cannot be vaccinated.
- Threshold: Varies by disease. Highly contagious pathogens like measles require >95% immunity to prevent outbreaks.
- Challenge: A decline in vaccination rates can cause herd immunity to fail, leading to re-emerging diseases.
The arrival of Europeans in Australia from 1788 introduced novel pathogens (like smallpox) into Aboriginal and Torres Strait Islander populations. Having been geographically isolated, they had no immunological memory, leading to catastrophic mortality rates (up to 90% in some areas). This demonstrates the devastating impact of pathogens on a non-immune population.
Immunotherapy: Monoclonal Antibodies (mAbs)
Immunotherapy harnesses the body’s immune system to fight disease. Monoclonal antibodies are lab-engineered proteins that can be used to target cancer cells. Toggle the switch to see how they can either flag a cancer cell for destruction or “release the brakes” on our T cells.
