Q.What is a vaccine ? Write the basis on which it acts when administered in the body.
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Start your 14-day free trial to unlock the full solution →A vaccine is a biological preparation containing antigens (weakened/killed pathogens or their parts) that stimulates the immune system to produce memory cells, conferring immunity without causing disease.
What is a vaccine?
A vaccine is a biological substance designed to train your immune system to recognize and fight specific pathogens—bacteria, viruses, or toxins—without actually making you sick. Think of it as a controlled rehearsal for your immune system: you expose it to a harmless version of the enemy so that when the real threat arrives, your body already knows how to respond swiftly and effectively.
Vaccines typically contain one of the following:
- Killed (inactivated) pathogens – the microbe is dead but its structure remains intact
- Weakened (attenuated) live pathogens – the microbe is alive but too weak to cause disease
- Subunit vaccines – only specific parts of the pathogen (proteins, polysaccharides)
- Toxoid vaccines – inactivated toxins produced by bacteria
- mRNA or viral vector vaccines – genetic instructions that make your cells produce a harmless piece of the pathogen
The key principle: the vaccine must carry antigens—molecular markers that the immune system can recognize as foreign.
The immunological basis: how vaccines work in the body
When a vaccine is administered, it triggers a sophisticated immune response that mirrors what would happen during a natural infection, but without the danger.
1. Antigen recognition and presentation
Once the vaccine enters the body (usually via injection), the antigens it carries are detected by antigen-presenting cells (APCs) like dendritic cells and macrophages. These cells engulf the antigens, process them, and display fragments on their surface using MHC (major histocompatibility complex) molecules. They then migrate to lymph nodes where the real action begins.
2. Activation of adaptive immunity
In the lymph nodes, APCs present the processed antigens to T-helper cells (CD4⁺ T cells). This interaction is the spark that ignites the adaptive immune response. The activated T-helper cells then:
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Stimulate B cells to proliferate and differentiate into:
- Plasma cells – antibody factories that secrete specific antibodies against the antigen
- Memory B cells – long-lived cells that "remember" the antigen
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Activate cytotoxic T cells (CD8⁺ T cells) which can kill infected cells (especially important for viral vaccines)
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Generate Memory T cells that persist for years
3. Primary immune response
The first exposure to the vaccine antigen produces a primary immune response. Antibody levels (mainly IgM, then IgG) rise over 1–2 weeks, peak, and then gradually decline. This response is slower and weaker than what will come later, but it's crucial because it establishes the memory cell population.
4. Immunological memory – the core principle
This is where the magic happens. The memory B and T cells generated during vaccination remain in the body for years, sometimes decades. They circulate in the blood and lymphatic system, constantly surveilling for their specific antigen. …
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