VI SINH Y HỌC | BÀI 6: ỨNG DỤNG MIỄN DỊCH TRONG VI SINH (02/10/2025)

Main Ideas

The lecture presents three major applications of immunology in microbiology: vaccination, antibody use for prevention or treatment, and antigen–antibody reactions for diagnosis.

1. Vaccines and Active Immunity

A vaccine introduces a microbial antigen—or a modified form of one—to stimulate the body to produce its own antibodies. The antigen must be prepared so it does not cause disease while retaining enough antigenic properties to provoke an immune response.

Vaccination produces artificial active immunity. It takes time for the body to recognize the antigen and produce antibodies. This protection generally lasts longer than protection from antibodies received from outside the body, although its duration varies and some vaccines require boosters.

The immune response depends on factors such as the amount and type of antigen, dose, route of administration, number of doses, and spacing between doses. These are established through research; more antigen does not necessarily produce proportionally more antibodies. The lecture emphasizes vaccinating ahead of an expected disease season so the body has time to develop protection.

Vaccine Types

• Inactivated (killed) vaccines: The pathogen is killed but may retain antigens that stimulate antibody production. The lecture describes these as generally safer than live vaccines, but often requiring multiple doses or boosters.
• Live attenuated vaccines: The pathogen is weakened so it has greatly reduced ability to cause disease while remaining capable of limited replication. This can produce a strong, longer-lasting response, but the lecture notes the possibility of the organism regaining virulence.
• Toxoid vaccines: A bacterial exotoxin is inactivated—for example, through formaldehyde treatment—so it no longer causes toxic effects but can still stimulate antibody production. The resulting antibodies, called antitoxins, neutralize the toxin rather than directly killing the bacterium. Toxoids are used for diseases such as tetanus and diphtheria.
• Other types: Subunit, polysaccharide, and conjugate vaccines are mentioned, along with vaccines developed using genetic and recombinant technologies.

A pathogen may have more than one vaccine formulation. Polio is one example: the lecture contrasts an inactivated vaccine with the oral live attenuated Sabin vaccine. The oral vaccine is described as stimulating protection at the intestinal entry site.

Herd Immunity and Vaccine Use

Herd immunity requires vaccination across a large share of a community. This reduces transmission and helps protect people who cannot be vaccinated or are especially vulnerable. The lecture contrasts a community with no immunity, where infection can spread readily, with one where many people are protected and transmission chains are interrupted.

Vaccine eligibility and schedules depend on the particular vaccine and target group. Examples from the lecture include childhood vaccination against diphtheria, pertussis, and tetanus; HPV vaccination; and shingles vaccination.

The lecturer stresses that no vaccine is entirely free of possible side effects, though not everyone experiences them. Reactions can range from local soreness or redness to systemic symptoms such as fever. The lecture gives fever and seizures as examples and quotes approximate rates; these rates are not universal and vary by vaccine.

The lecturer encourages weighing the risks of side effects against the risks of infection and onward transmission, especially to vulnerable people.

2. Antibodies for Prevention and Treatment

Giving antibodies made outside a patient produces artificial passive immunity. Protection is available quickly, but it is temporary because the recipient’s body did not make the antibodies itself. The lecture gives roughly 15–21 days as an example of how long passively administered antibodies may persist, while noting that duration depends on the antibody.

Antibodies may be obtained from:

• Human sources, such as people who have recovered from an infection. The lecture describes these as less likely to cause allergic reactions but costly.
• Animal sources, such as horses. These may be less expensive, but animal proteins can trigger allergic reactions; the lecture notes that testing may be needed before administration.

Tetanus Example

Tetanus bacteria can produce an exotoxin in a wound. The toxin can affect the nervous system, causing muscle rigidity and spasms. Severe involvement of muscles needed for breathing can be fatal.

The lecture distinguishes between:

• Treatment: Antitoxin may be given when tetanus is suspected or present to neutralize toxin still circulating freely. The lecturer stresses that antibodies cannot neutralize toxin once it has bound to nervous-system cells.
• Post-exposure prevention: After a wound that may be contaminated with tetanus spores, tetanus antitoxin may be given when indicated, even if symptoms have not appeared. The purpose is to neutralize toxin if the spores germinate and toxin is produced.

These are examples from the lecture, not individualized treatment guidance.

3. Antigen–Antibody Reactions in Diagnosis

Antigens and their corresponding antibodies bind specifically. This specificity can be used to identify infectious agents.

The lecture describes two basic diagnostic approaches:

• Detecting an antigen: Add a known antibody that recognizes the suspected antigen. If a specific antigen–antibody complex forms, that supports the presence of the antigen in the sample.
• Detecting an antibody: Add a known antigen to the patient’s serum. If a complex forms, the corresponding antibody is present.

A positive antibody result does not by itself prove an active, current infection. It may reflect a past infection or vaccination. Further interpretation may involve the antibody class—such as IgM versus IgG—and whether antibody levels are rising over time.

The lecture introduces antibody titre as the greatest dilution of serum that still produces a detectable, specific reaction. Comparing titres can help assess changes in antibody levels.

Timing matters. Early in an illness, testing for the pathogen or its antigen may be useful. Later, when the pathogen or antigen is no longer detectable in the sample, antibody testing may be more informative.

Cerebrospinal fluid is used as an example: it is normally sterile, so detecting an appropriate microbial antigen in a sample can support a diagnosis of infection.

Named reaction types include agglutination, precipitation, neutralization, complement fixation, immunofluorescence, enzyme-linked assays (such as ELISA), and radioimmunoassays. They differ in how the resulting antigen–antibody complex is detected—for example, through visible clumping, precipitate formation, or a fluorescent or enzyme-based signal.

Speakers and Sources Featured

• Speaker: One unnamed instructor or lecturer who leads the lesson and explains the concepts.
• Other voices: Students in the classroom whose brief answers are prompted by the lecturer; they are not individually identified.
• Sources or references mentioned: A textbook and its tables. The lecturer also refers to the Ministry of Health when discussing COVID-19 vaccine approvals. No individual authors or officials are named.