Glossary: Immunity in Humans

This chapter's terms in context

Chapter 11 is organised around five terms that describe how the human body recognises and resists disease-causing organisms: pathogen, antigen, antibody, immunity and vaccine. A pathogen is any microorganism capable of causing disease in a host, including bacteria, viruses, fungi and protozoa, and the body's non-specific defences, the skin, mucus, cilia and stomach acid, form the first line of protection against pathogens before the immune system's specific response is even needed. An antigen is a specific molecule, usually a protein found on the surface of a pathogen or foreign cell, that the immune system recognises as foreign and that triggers an immune response; every pathogen carries its own distinctive antigens, which is exactly why the immune response mounted against one pathogen is normally specific to that pathogen alone.

An antibody is a Y-shaped protein produced by white blood cells (specifically lymphocytes) in response to a particular antigen, and each antibody has a shape that binds specifically to one particular antigen, in the same lock-and-key relationship used to describe enzyme specificity in an earlier chapter. Once bound, antibodies can neutralise pathogens directly, cause them to clump together (agglutination), or mark them for destruction by other white blood cells through phagocytosis. A very common exam error is confusing antigen and antibody: the antigen is the foreign molecule that triggers the response and comes from the pathogen, while the antibody is the defensive protein the body itself produces in response, reversing these two in an answer is treated as a serious conceptual error, not a minor slip.

Immunity is the body's overall ability to resist a specific pathogen, usually because antibodies against that pathogen's antigens are already present or can be produced rapidly, and it is classified along two independent dimensions: natural versus artificial (depending on whether exposure was unplanned or deliberately induced) and active versus passive (depending on whether the body's own lymphocytes produced the antibodies, giving long-lasting protection, or whether ready-made antibodies were supplied from an outside source, giving only temporary protection). A vaccine works by introducing a weakened, killed or partial form of a pathogen's antigens into the body, deliberately triggering an active immune response and memory cell production without causing the actual disease, so that on future exposure to the real pathogen, antibodies can be produced fast enough to prevent illness, a form of artificial active immunity, and questions frequently ask a candidate to place vaccination correctly within the four-way classification of natural/artificial and active/passive immunity rather than simply describing what a vaccine does.

Because these five terms depend on one another in sequence, a pathogen carries an antigen, an antigen triggers antibody production, antibody production combined with memory cells gives immunity, and a vaccine is a deliberate, engineered way of inducing that immunity safely, losing the correct order of this chain is the single most common way marks are lost across the whole chapter, more so than forgetting any one definition in isolation.

Paper 2 questions on this chapter frequently present an unfamiliar scenario, a person recovering from an infection, or a population being vaccinated against a new disease, and ask a candidate to explain, using the correct terms, why immunity develops or why a second infection by the same pathogen produces a milder response. The strongest answers explicitly mention memory cells, produced during the first exposure, that allow a faster and larger antibody response on re-exposure to the same antigen, rather than vaguely stating that the body 'remembers' the disease. A frequent scoring gap is describing active immunity correctly in principle but forgetting to specify whether the scenario given is natural (through actual infection) or artificial (through vaccination), a distinction examiners consistently expect to be stated rather than implied.