Question
Define the term vaccine. Describe the different types of vaccines and explain how they confer immunity.
(CBSE 2024 Board Exam — 5 marks)
Solution — Step by Step
A vaccine is a biological preparation that introduces an antigen into the body to stimulate the immune system — without causing the actual disease. The body mounts a primary immune response, produces memory B and T cells, and stays prepared for a future real infection.
Active immunity — the body produces its own antibodies after exposure to an antigen (through infection or vaccination). It takes time to develop but is long-lasting.
Passive immunity — preformed antibodies are transferred directly into the person. This works immediately but fades within weeks because the body never made those antibodies itself. Classic examples: maternal antibodies passed to a foetus via the placenta, or anti-snake venom serum injected after a snakebite.
| Type | What it contains | Example |
|---|---|---|
| Live attenuated | Weakened (but alive) pathogen | BCG (TB), OPV (Polio), MMR |
| Killed/Inactivated | Dead pathogen | Salk polio vaccine, Rabies vaccine |
| Subunit / Toxoid | Only antigen or inactivated toxin | Hepatitis B, Tetanus toxoid |
| Recombinant DNA | Antigen produced by a host organism | Hepatitis B (yeast-produced), COVID-19 mRNA vaccines |
After vaccination, antigen-presenting cells (APCs) process the foreign antigen and present it to B lymphocytes and T lymphocytes. B cells differentiate into plasma cells that secrete antibodies. Crucially, some B and T cells become memory cells — these persist for years and can mount a rapid secondary immune response if the real pathogen enters later.
Vaccines work by mimicking infection without disease, triggering active immunity and creating immunological memory. The body is primed — the next time the real pathogen arrives, the response is so fast the person never develops symptoms.
Why This Works
The brilliance of vaccination lies in the clonal selection theory. Each B cell has receptors for one specific antigen. When a vaccine antigen binds to the matching B cell, that cell rapidly divides — clonal expansion. Most clones become antibody-secreting plasma cells; a few become memory cells.
Live attenuated vaccines produce the strongest immunity because they replicate briefly inside the body, giving the immune system a richer, longer exposure to multiple antigens. That’s why BCG is a one-dose vaccine. Killed vaccines are safer (no risk of reversion to virulence) but generally need booster doses because the immune stimulation is weaker.
Toxoid vaccines are a clever workaround for diseases like tetanus where the bacterium itself isn’t the problem — the toxin is. Formaldehyde-inactivated toxin (toxoid) can’t cause disease but still raises antibodies against it.
Alternative Method — Active vs Passive Table (Quick Answer Format)
For a 3-mark question asking only to distinguish active and passive immunity:
| Feature | Active Immunity | Passive Immunity |
|---|---|---|
| Antibody production | By the individual | Transferred from outside |
| Onset | Slow (days–weeks) | Immediate |
| Duration | Long-lasting (years) | Short-lived (weeks) |
| Memory cells | Formed | Not formed |
| Example | Vaccination, natural infection | Maternal antibodies, antiserum |
In NEET, “which type of immunity involves memory cells?” is a repeat question. The answer is always active immunity — passive immunity borrows antibodies and leaves no memory behind.
Common Mistake
Students write that all vaccines give lifelong immunity — this is wrong. Killed vaccines (like the influenza vaccine) often need annual boosters because immunity wanes. Live attenuated vaccines generally give longer-lasting immunity. Don’t generalise; mention the type of vaccine when writing about duration of immunity.
A second slip: confusing antigen and antibody in the definition. A vaccine introduces an antigen (or something that acts like one). The body then produces antibodies in response. Never say “a vaccine introduces antibodies.”