Innate defence acts first
Innate immunity includes physical and chemical barriers as well as rapid internal responses. Skin, mucus, cilia, stomach acid, lysozyme and normal microbiota reduce pathogen entry. If a barrier is crossed, phagocytes, complement proteins, inflammatory mediators and natural killer cells respond.
Innate receptors recognise broad molecular patterns shared by groups of microbes. The response is rapid and does not improve through antigen-specific memory in the way adaptive immunity does.
| Feature | Innate | Adaptive |
|---|---|---|
| Speed | Minutes to hours | Slower during first exposure |
| Recognition | Shared microbial or damage patterns | Highly specific antigens |
| Main cells | Phagocytes, NK cells and others | B and T lymphocytes |
| Memory | No classical antigen-specific memory | Strong memory after activation |
Inflammation recruits and contains
Tissue damage and microbial signals cause local release of mediators such as histamine. Blood vessels dilate and become more permeable, increasing delivery of plasma proteins and immune cells. Redness, heat, swelling and pain are consequences of these vascular and chemical changes.
Neutrophils and macrophages engulf material by phagocytosis. Vesicles containing a microbe fuse with lysosomes, exposing it to enzymes and reactive chemicals. Macrophages and dendritic cells can also present antigen fragments to T lymphocytes, linking innate detection with adaptive activation.
Pathogen or damage signals are detected.
Blood flow and permeability increase.
Innate cells engulf and process material.
Specific T cells are activated.
Effector and memory cells expand.
Clonal selection creates specificity
Each lymphocyte carries receptors with one main antigen specificity. An antigen selects the rare lymphocytes whose receptors fit. With the required co-stimulation, those cells proliferate to form a clone.
Some descendants become short-lived effector cells; others become long-lived memory cells. Helper T cells coordinate responses through direct contact and cytokines. Cytotoxic T cells kill infected body cells displaying relevant antigen, while activated B cells can differentiate into antibody-secreting plasma cells.
Antibodies label with precision
An antibody has variable regions that form antigen-binding sites and a constant region that interacts with other immune components. Antibody binding can neutralise toxins or viruses, agglutinate particles, mark targets for phagocytosis and activate complement.
Antibodies act mainly against extracellular targets and free viral particles. They do not enter every infected cell to remove intracellular pathogens; cytotoxic T cells are especially important for that problem.
B cells and antibodies target extracellular material.
T cells coordinate or kill infected cells.
Both arms generate faster future responses.
Memory explains vaccination
The primary response to a new antigen takes time because rare specific cells must be activated and multiplied. A second exposure activates memory cells more rapidly and usually produces a faster, larger and more sustained response.
Vaccination produces active immunity by exposing the immune system to a safe form or component of an antigen. Passive immunity transfers ready-made antibodies, giving immediate but temporary protection without the same memory-cell development.
Follow an antigen through a coordinated response
After a barrier is crossed, innate recognition and local inflammation recruit defensive cells. Antigen-presenting cells help activate lymphocytes whose receptors recognise suitable antigen. Matching cells expand: the pathogen does not instruct all lymphocytes to manufacture one new receptor.
Some activated B cells become antibody-secreting plasma cells, while other adaptive cells coordinate responses or kill infected host cells. Antibodies act on accessible antigens and do not simply enter every infected cell to remove its viral genome. Following control of the antigen, memory populations can remain. Re-exposure can then produce a shorter lag and stronger response, but specificity and persistence vary. Innate mechanisms continue cooperating throughout.
The ideas to carry forward
- Innate defences are rapid and pattern-based.
- Adaptive responses depend on antigen-specific clonal selection.
- B cells support antibody-mediated defence; T cells coordinate and kill infected cells.
- Memory cells make later responses faster and stronger.
Answer first. Then reveal the marking logic.
01Why does passive immunity not usually produce long-term immune memory?2 marks · show the biological link
Answer: The recipient receives antibodies rather than activating and cloning their own antigen-specific lymphocytes.
02How can a phagocyte help start an adaptive response?2 marks · show the biological link
Answer: It processes engulfed material and presents antigen fragments that contribute to activation of specific T lymphocytes.
03What is clonal selection?2 marks · show the biological link
Answer: Activation and expansion of lymphocytes with receptors matching an antigen.
04Why can the later response start faster?2 marks · show the biological link
Answer: Antigen-specific memory populations already exist.
05Are antibodies sufficient for every infected cell?2 marks · show the biological link
Answer: No. Cell-mediated mechanisms also contribute.
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