Human Health and Disease · Lesson 5 of 11
Active and Passive Immunity, Vaccines and Immunisation
“Antigen-induced memory, maternal antibody transfer, vaccination, recombinant proteins and DNA vaccine principles.”
• Explain active immunity through antigen-driven activation. • Distinguish passive antibody transfer from memory-generating immunity. • Describe the immunological basis of vaccination. • Explain recombinant hepatitis B surface-antigen production. • Describe what makes a DNA vaccine gene biologically suitable.
Active immunity is produced by the recipient
Active immunity develops when the body encounters an antigen and mounts its own antigen-specific response. The antigen may be introduced naturally during infection or intentionally through a vaccine. Antigen-presenting cells and other immune signals contribute to lymphocyte activation, and selected B and T cells proliferate to form effectors and memory populations. This response requires time because the recipient must recognise the antigen and build the relevant cellular response. Its advantage is that memory can remain after the initial antigen has been removed.
Vaccination takes advantage of immunological memory. Depending on the vaccine, the body may receive inactivated or weakened microorganisms, selected antigens, or molecular instructions that allow host cells to produce an antigen. The immune system responds to that antigen without necessarily experiencing the disease associated with the full infectious organism. Booster doses can strengthen or restore responses where needed, although some vaccines offer durable protection without the same booster schedule. Effectiveness depends on antigen variation, host immunity and the particular outcome being measured.
Passive immunity supplies antibodies made elsewhere
Passive immunity occurs when ready-made antibodies are transferred to an individual. Maternal IgG crosses the placenta during pregnancy, providing temporary protection against some infections in the newborn. Secretory IgA in colostrum and breast milk helps protect mucosal surfaces. Clinical passive immunisation may include antitoxin or immune-globulin preparations for selected exposures, including certain toxins or venom. Because the antibodies are already formed, their action can be rapid and does not require the recipient to expand a new antigen-specific clone first.
Transferred antibodies are gradually broken down or removed, so passive immunity is normally temporary. Receiving antibodies alone does not establish a durable population of the recipient's own memory B and T cells. Active and passive approaches can therefore complement each other rather than replace each other: in some high-risk exposure situations, clinicians may use preformed antibodies for immediate protection and a vaccine for longer-term immunity. The source of the protective molecules is the key distinction.
Recombinant proteins and genetic vaccine approaches
Recombinant DNA methods allow production of a selected pathogen antigen in engineered cells. A gene encoding the antigen can be introduced into a production organism such as yeast; after expression, the resulting protein is isolated and formulated for vaccination. The recombinant hepatitis B vaccine uses hepatitis B surface antigen as a major protective target. It is not equivalent to giving the recipient a complete replicating hepatitis B virus. This approach demonstrates how knowledge of protein structure and gene expression supports disease prevention.
A DNA vaccine carries genetic information designed to express a selected antigen in the recipient's cells. A suitable gene is one encoding an antigen whose expression is safe enough for the intended approach and likely to stimulate useful specific recognition. It should be chosen because the protein helps elicit protective immune responses, not because the sequence increases the pathogen's harmfulness. DNA vaccines and mRNA vaccines use different molecular delivery and expression pathways, even though each may lead to antigen production and immune activation.
Vaccination as part of public-health prevention
Vaccination has controlled many infectious diseases and was central to the eradication of smallpox. Its population effects depend on how strongly it reduces infection, severe illness or onward spread, how widely it is administered, and whether the pathogen evolves antigenic differences. Vaccination is not a universal substitute for clean water, safe food, respiratory hygiene or mosquito control, because different pathogens enter through different routes. An effective prevention strategy addresses the particular agent's life cycle as well as the host's immune susceptibility.
| Property | Active immunity | Passive immunity |
|---|---|---|
| Origin | Host makes the response | Preformed antibodies are transferred |
| Example | Infection or vaccination | Maternal IgG and milk IgA |
| Onset | Requires activation and expansion | Often acts relatively rapidly |
| Memory | Can produce memory cells | Transfer alone produces no recipient memory |
| Duration | Varies; may be long-lasting | Limited by survival of transferred antibodies |
Quick Quiz
Quiz
How does active immunity arise?
Which is natural passive immunity?
Why does passive immunity usually fade?
What principle explains vaccination effectiveness?
Which illustrates a recombinant protein vaccine?
What should a DNA vaccine gene encode?
Practice Problems
- Differentiate naturally acquired active immunity, vaccine-induced active immunity and maternal passive immunity.
- Explain the production principle of a recombinant hepatitis B vaccine and why it is not infectious hepatitis B virus.
- Explain the phrase suitable gene when discussing DNA vaccines.
- Describe why active and passive protection may both be useful in a high-risk exposure scenario.
Naturally acquired active immunity follows infection-related antigen exposure and stimulation of the person's own immune system, sometimes producing lasting memory but also disease. A vaccine introduces selected antigenic material or instructions in a controlled preventive context, aiming to generate specific protection without requiring full infection. Passive immunity results from transfer of existing antibodies, such as maternal IgG through the placenta or IgA in milk. It acts without requiring the recipient to manufacture those antibodies, but the transferred molecules eventually decline and do not by themselves establish memory.
Key Takeaways
• Active immunity involves host-generated antigen-specific responses and can create memory. • Passive immunity transfers existing antibodies and usually acts more rapidly. • Maternal IgG and milk IgA are natural forms of passive protection. • Vaccines use antigen exposure to generate immune memory. • Recombinant vaccine antigens can be produced using engineered cells. • DNA vaccines carry instructions to make an antigen, not preformed antibodies. • Vaccination complements environmental and community disease-control measures.