ANSC20003 Chap.10 Fundamentals of the Immune System
Fundamentals of the Immune System
Detect, defend, remember
Read it as one system assembled from several parts: body surfaces, soluble molecules, several cell lineages, and the tissues that make and house them. It notices microbes, foreign material and injured tissue. It keeps invaders out and clears the ones that get in, along with any cell taken over or gone wrong. It keeps its fast and slow arms working to one plan.
It holds inflammation inside limits and hands the site over for repair. And it keeps a record.
Four verbs organise the whole week: recognise, respond, regulate and remember.
Barriers, and where pathogens actually enter
Body surfaces defend physically, through unbroken skin, the epithelium of every mucosal surface, the junctions sealing those cells together and a moving blanket of mucus; chemically, through acidity, enzymes such as lysozyme, short antimicrobial peptides and iron-binding proteins; and mechanically, through a cough or sneeze, the flushing of tears and urine, gut movement and the constant loss of surface cells.
The resident microbiota is listed alongside them.
The entry sites include the respiratory, gastrointestinal and urogenital tracts, skin wounds and vector bites, the conjunctiva, the mammary gland through the teat canal, the placenta, and the umbilicus in newborns, and the last three are what make this a veterinary rather than a generic account.
Innate recognition and the cells that act on it
Pattern recognition receptors detect conserved microbial structures and host molecules released by stressed or dying cells, and the output is cytokine production, inflammation and antimicrobial responses.
Neutrophils phagocytose rapidly, macrophages phagocytose and repair and present antigen, dendritic cells capture antigen and activate T cells, natural killer cells kill some infected or abnormal cells, mast cells release mediators quickly, and eosinophils act against helminths.
Complement, antimicrobial molecules and type I interferons are the soluble arm, and cytokines are the signalling layer over all of it.
Two ways of seeing an antigen
B-cell receptors read accessible regions of intact antigen, while T-cell receptors read peptides displayed in MHC molecules, which is why something has to process the antigen first.
Helper T cells coordinate, cytotoxic T cells kill infected cells, and B cells become antibody-secreting plasma cells and memory cells. Adaptive immunity is defined by specificity, diversity, tolerance and memory, and protection divides into active immunity, which is earned and remembered, and passive immunity, which is transferred and temporary.
What this chapter covers
- 01
Barriers, innate and adaptive immunity as three layers separated by time
- 02
Physical, chemical and mechanical barriers, and the resident microbiota as part of them
- 03
The entry sites that matter in animals, including the teat canal and the umbilicus
- 04
Pattern and damage recognition, and what pattern recognition receptors trigger
- 05
The innate cell types and their specialised jobs
- 06
Complement, antimicrobial molecules, interferons, and cytokines as the signalling layer
- 07
How B cells and T cells see antigen differently, and active against passive immunity
Explaining an infection that persists despite a high titre
- 1Explain why antibody cannot resolve this infection.
- 1Name the arm of immunity that can, and the cells involved.
Key terms
- Innate immunity
- The rapid arm of defence that recognises conserved microbial patterns with a limited receptor set and generates no classical antigen-specific memory.
- Pattern recognition receptor
- A receptor that detects conserved microbial structures or host molecules released by damaged cells, triggering inflammation.
- Dendritic cell
- The professional antigen-presenting cell that captures antigen in tissue and activates naive T cells, linking the two arms of immunity.
- Immunological tolerance
- The property that normally prevents adaptive responses against the animal's own antigens.
- Passive immunity
- Protection transferred to an animal ready-made, which is temporary because the recipient produces nothing and retains no memory.
Fundamentals of the Immune System FAQ
What is the practical difference between innate and adaptive immunity?
Innate immunity is rapid, works on microbial patterns with a limited receptor repertoire and has immediate effector mechanisms but no classical memory. Adaptive immunity is slower on first exposure, recognises specific antigens with an extremely diverse receptor set, and generates memory. The innate response holds the line while the adaptive one develops.
Why do B cells and T cells need antigen presented differently?
A B-cell receptor reads accessible regions of intact antigen directly. A T-cell receptor reads short peptides held in MHC molecules, so the antigen has to be captured and processed by another cell first. That is why dendritic cells are described as the link between the two arms.
Which barrier is easiest to improve on a farm?
The first layer. Ventilation, bedding, hygiene at milking and umbilical care all act on physical and chemical barriers or on the entry sites themselves, and they work against every pathogen at once rather than against one agent.
What does tolerance mean in this context?
It is one of the four defining properties of adaptive immunity, alongside specificity, diversity and memory, and it is the property that normally prevents responses against the animal's own antigens. Its failure is one of the four ways disease damages tissue.
Exam move
Build the three layers as a timeline first and hang every component off it, because the timing is what distinguishes them and what questions test. Learn the innate cells by their single distinctive job rather than by morphology. For the recognition section, rehearse the B cell against T cell contrast until it is one sentence, since it underpins the diagnostics chapters as well.
Practise placing a pathogen in one of three locations and naming the arm that reaches it, as that pairing recurs across the immunology block.
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