Every fact in the three immune system lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications and care points: this is general science education, not medical advice. The immune system has many more cell types and signals than the lessons show; we group them into barriers, the fast innate response and the slower adaptive response, as school textbooks do. Timings such as 'days or even weeks' for a first adaptive response are general textbook statements and vary between infections and people. Glitter or paint stands in for germs in the experiments: it shows how things spread and wash off, but real microbes are far smaller and behave differently. Pupils should not compare their own health or immune system with others; questions about a pupil's health, allergies or medicines go to a parent, the school nurse or a doctor.
All sources
[1] Overview of the Immune System
National Institute of Allergy and Infectious Diseases (NIH, USA) · Reliability: High
Type: National health research agency
The overall function of the immune system is to prevent or limit infection; it can tell normal, healthy cells from unhealthy ones. All immune cells come from precursors in the bone marrow; T cells mature in the thymus in the upper chest; lymph nodes are communication hubs; the spleen, behind the stomach, processes information from the blood. White blood cells (immune cells) circulate in the blood, patrolling for problems. Neutrophils and macrophages are first-line responders (innate); B and T cells respond to specific microbes based on previous encounters (adaptive), using immunological memory.
Innate immunity is non-specific and is not caused by an infection or vaccination. Skin contains keratin that resists entry; sticky mucus traps pathogens; saliva, mucus and tears contain an enzyme that breaks down bacterial cell walls; stomach acid kills many pathogens. Macrophages engulf foreign particles and pathogens; neutrophils arrive early and engulf and digest pathogens. Inflammation causes local redness, swelling, heat and pain; fever inhibits the growth of pathogens. Natural killer cells kill virus-infected cells. The adaptive system takes longer to respond.
Macrophages and dendritic cells engulf pathogens and debris by phagocytosis; neutrophils use lysosomes to digest engulfed pathogens. Dendritic cells present antigens on their surface, triggering adaptive immunity. T cells mature in the thymus and B cells in the bone marrow. Neutrophils make up 50–60% of all leukocytes (white blood cells).
Adaptive immunity occurs after exposure to an antigen from a pathogen or a vaccination, and takes longer to develop than innate responses. An antigen is a molecule that stimulates an immune response. B cells make antibodies, proteins produced by plasma cells, which bind the antigen. Helper T cells release cytokines; cytotoxic T cells destroy infected cells. Memory cells are antigen-specific B or T lymphocytes that respond quickly on re-exposure. Vaccination uses non-infectious antigens from known pathogens to produce a mild primary response and immune memory.
The adaptive response takes days or even weeks to become established. B cell receptors (membrane-bound antibodies) are Y-shaped with two heavy and two light chains; the variable region binds the antigen. Activated B cells become plasma cells that secrete antibodies with the same antigen recognition. Memory cells on re-exposure output tens to hundreds-fold greater antibody amounts than in the primary response. A person has hundreds of millions of different B cells.
Centers for Disease Control and Prevention (USA) · Reliability: High
Type: National public health agency
Washing hands can keep you healthy and prevent the spread of respiratory and diarrhoeal infections. Germs spread when people touch their eyes, nose and mouth with unwashed hands, prepare or eat food with unwashed hands, touch dirty surfaces, or cough or sneeze into their hands and then touch others. Steps: wet, lather (backs of hands, between fingers, under nails), scrub for at least 20 seconds, rinse, dry.
Centers for Disease Control and Prevention (USA) · Reliability: High
Type: National public health agency
A good lather forms pockets called micelles that trap and remove germs, harmful chemicals and dirt. Studies show you need to scrub for 20 seconds to remove harmful germs and chemicals; shorter washing removes fewer. Warm and cold water remove the same number of germs. Hand sanitizer is for hands that are not visibly dirty or greasy.