The Immune System: A Very Short Introduction

Citation

Klenerman, P. (2025). The Immune System: A Very Short Introduction (2nd ed.). Oxford University Press.

Chapter Summary

The second edition was published online by Oxford University Press in August 2025 and in print in September 2025. It has eight chapters, preceded by a note on the new edition, a list of illustrations and a list of abbreviations. Oxford's description sets out its aims: to explain how the immune system evolved and the principles that guide its responses, the role of immune memory and vaccines, what happens when there is too little or too much immunity, and what the COVID-19 pandemic revealed. The pandemic chapter is the clearest addition to this edition.

Chapter 1: What is the immune system?

The book opens by presenting the immune system as an evolved network that protects against both outside pathogens and inside threats such as cancer. It compares strategies across organisms, from CRISPR/Cas defences in bacteria to pattern recognition in animals. It then describes the human system: the thymus, lymph nodes and spleen, the barriers of skin and mucous membranes, and soluble mediators such as cytokines, chemokines and complement. It ends by contrasting innate and adaptive immunity.

Chapter 2: First responders: the innate immune response

This chapter explains how innate immunity senses danger through receptors for pathogen-associated and damage-associated molecular patterns. Macrophages, dendritic cells and neutrophils launch rapid defences, helped by cytokines, interferons and complement. The chapter compares responses to viruses and bacteria, describes the body-wide acute phase response, and shows how pathogens evade detection. It also introduces the links between innate and adaptive immunity, and their importance for vaccine design.

Chapter 3: Adaptive immunity: a voyage of (non-)self-discovery

Here the question is how adaptive immunity achieves enough diversity to meet changing threats such as SARS-CoV-2. B cells generate a wide antibody repertoire by recombining V, D and J gene segments. T cells use a similar mechanism to detect infected cells through peptides presented by MHC molecules. The chapter describes the roles of CD8+ killer T cells and CD4+ helper T cells, and introduces unconventional T cells with specialised functions in tissues.

Chapter 4: Making memories

This chapter follows a response from innate activation through the priming, expansion and differentiation of T and B cells, to the formation of memory. Long-lived antibody-producing cells reside in the bone marrow, and memory T cells divide into central, effector and tissue-resident populations. Vaccines are presented as imitations of infection that induce similar memory. The chapter covers conjugate vaccines, mRNA vaccines and passive immunisation.

Chapter 5: Too little immunity: immunological failure

Immune failure is explored through genetic defects and HIV. The chapter shows how redundancy can hide some defects, while others, such as loss of the common gamma chain, cause severe combined immunodeficiency. It discusses MHC diversity as a source of individual differences in response to infection and vaccines, as well as malnutrition and viral attack. HIV's evasion strategies are examined: mutation, latency and immune exhaustion. The chapter ends with the limits of treatment and the search for a cure or vaccine.

Chapter 6: Too much immunity: auto-immunity and allergic diseases

Overactive immunity is explained as a failure of tolerance. Central tolerance in the thymus, including the role of the AIRE gene, removes self-reactive T cells. Peripheral mechanisms, such as regulatory T cells and inhibitory receptors, hold others in check. The liver and the placenta are discussed as sites that actively induce tolerance. Rheumatoid arthritis, multiple sclerosis and inflammatory bowel disease illustrate how genes and environment, including microbes, combine to cause disease. Allergy is treated as a maladaptive type 2 response.

Chapter 7: Immunology taught by COVID-19

The new chapter uses SARS-CoV-2 as a case study. It covers the virus's evasion strategies, the protective and harmful roles of the immune response, and the development of mRNA and adenoviral vaccines. It also discusses findings on interferon signalling, cross-reactive T cells and variants that escape neutralising antibodies, along with rare vaccine side effects and the effect of dosing intervals. It closes with the role of global scientific collaboration in diagnostics, treatment and vaccination.

Chapter 8: The immune system v2.0: biological and immune therapies

The final chapter surveys how immunology is changing medicine in three ways: boosting responses with vaccines, directing immunity against cancer, and suppressing autoimmunity with biological drugs. It explains why vaccines against HIV, malaria and tuberculosis are hard to make. It describes checkpoint inhibitors, CAR-T cells and monoclonal antibodies against inflammatory pathways, and ends with the decline of immunity in old age and interventions aimed at restoring it.

Key Concepts

Chapters 1–2: What is the immune system?; First responders

• Innate and adaptive immunity: an immediate, pattern-based defence, followed by a slower, highly specific response with memory.
• Pattern recognition receptors: sensors for molecular signs of pathogens or tissue damage.
• Soluble mediators: cytokines, chemokines, interferons and complement, which coordinate the response.
• Acute phase response: body-wide physiological changes during infection.

Chapters 3–4: Adaptive immunity; Making memories

• V(D)J recombination: shuffling of gene segments to generate diverse antibodies and T-cell receptors.
• MHC presentation: display of peptide fragments on cell surfaces, which allows T cells to detect infection inside cells.
• Immunological memory: long-lived B cells, plasma cells and memory T-cell subsets formed after a first encounter.
• Vaccination: inducing memory without disease, using conjugate, mRNA or other platforms.

Chapters 5–6: Too little immunity; Too much immunity

• Immunodeficiency: failure caused by inborn errors, malnutrition or viruses such as HIV.
• Tolerance: central deletion of self-reactive T cells (helped by AIRE) and peripheral control by regulatory T cells.
• Autoimmunity and allergy: diseases caused by misdirected or excessive responses, shaped by genes and environment.

Chapters 7–8: COVID-19; Biological and immune therapies

• Immune evasion and variants: viral strategies that escape interferons and antibodies.
• Cross-reactive T cells: T cells primed by earlier infections that recognise a new virus.
• Immunotherapy: checkpoint inhibitors, CAR-T cells and monoclonal antibodies.
• Immune ageing: the decline of immune function with age.

Critical Analysis

Strengths

• Up to date. The 2025 edition adds a full chapter on what COVID-19 taught immunologists. Chapters 3 and 4 already draw on SARS-CoV-2 and mRNA vaccines as examples.
• Built around a few clear questions. The chapter titles move from what the system is, through how it responds and remembers, to what happens when it does too little or too much. This gives a non-specialist a framework to hang details on.
• An evolutionary and comparative opening. Chapter 1 begins with defences in bacteria and other organisms before turning to humans. It presents immunity as a general biological problem, not only a medical one.
• Medical relevance throughout. Immunodeficiency, HIV, autoimmune diseases, allergy, cancer immunotherapy and immune ageing are all covered. Readers interested in medicine will see why the basic mechanisms matter.

Limitations

• Very compressed. Eight short chapters cover ground that course textbooks spread over twenty or more. Topics such as antigen processing, lymphocyte development and the regulation of responses can only be outlined.
• A general-reader book. The series is designed to introduce a subject quickly. It is not organised for systematic study in the way a course textbook is, and it cannot be relied on to prepare for an examination.
• Diseases by example. Disease is covered through a few chosen cases rather than surveyed. Immune failure is examined mainly through genetic defects and HIV. Autoimmunity is examined through rheumatoid arthritis, multiple sclerosis and inflammatory bowel disease. Students who need a systematic list of conditions must turn to a textbook.

Where It Fits in the Course

This is the shortest and most accessible book on the immunology list, and it works best as a first read or a companion. Richard Coico's Immunology: A Short Course and Peter Parham's The Immune System are concise textbooks for a first semester course. Reading Klenerman first gives the overall logic of innate and adaptive immunity, memory and immune failure before those books add cellular and molecular detail. Klenerman's chapters on COVID-19 and on biological therapies are also useful updates to read alongside them.

For deeper study, Kuby Immunology teaches the field through experiments and data-based problems. Janeway's Immunobiology offers a more detailed account for advanced undergraduates and graduate or medical students. Paul's Fundamental Immunology is a multi-author reference for research-level questions. None of these is needed to read Klenerman, and Klenerman cannot substitute for them in a course with examinations.

How to Use This Book

School biology is enough background, and a list of abbreviations is included. The book suits students about to start immunology, students in neighbouring subjects such as microbiology, pharmacology or medicine who want an overview, and general readers.

• Read in order: Chapters 1–4 build the basic model (defences, recognition, memory) and are the core.
• Then the clinical chapters: Chapters 5 and 6 apply the model to immunodeficiency, autoimmunity and allergy. They are the most useful preparation for a disease-oriented course.
• Current topics: Chapters 7 and 8 can be read as a pair on vaccines and immune-based therapies. Readers pressed for time could read Chapter 8 alone.
• Alongside a textbook: when a course reaches innate immunity, antigen receptors, memory or tolerance, reread the matching Klenerman chapter first for the big picture, then turn to the textbook for mechanisms.

Keeping a one-page diagram of the main cell types, to be filled in as each chapter adds to it, helps fix the overall structure.