Types of immunity

Immunity can be classified as natural or artificial, and as active or passive. Explain what each of these four terms means and give an example of each possible combination.

The table in the diagram classifies types of immunity according to two criteria. Place each of the following situations in its corresponding box (1 to 4) and justify your choice: a) contracting measles; b) receiving the tetanus vaccine; c) receiving antivenom serum after a snake bite; d) a baby being breastfed.

What is a vaccine? Explain why vaccination is an example of artificial active immunity.

Vaccines can contain attenuated microorganisms, inactivated ones, fragments of the pathogen or toxoids. Explain what all of them have in common and why it is essential that they do not cause the complete disease.

What does an immune serum contain (obtained from a person or an animal already immunised)? Explain why its administration is considered passive immunity.

Tetanus antiserum contains pre-formed antibodies. Explain why its protection is immediate but short-lived, and why the vaccine is nonetheless also administered during the same visit.

A hiker is bitten by a viper in the mountains. In the emergency room, they immediately administer heterologous hyperimmune antivenom serum (obtained from a horse), and he recovers successfully. Eight months later, the same hiker suffers a second bite from the same viper species.

1. a) Predict whether the patient will be protected against the venom in this second episode without receiving new treatment. Justify your answer by analysing the cellular and humoral mechanisms that were activated (or were not activated) following the administration of the serum in the first bite.
2. b) Describe how the plasma concentration of specific antibodies against the venom would vary over time after the first bite treated with serum, and compare it with the curve that would have been obtained if a vaccine (harmless toxoid) had been used.
3. c) Justify why the repeated administration of serum obtained from another animal species carries a serious immunological risk for the patient starting from the second dose.

What is herd immunity (or collective immunity)? Explain why it is important for a high percentage of the population to be vaccinated, even if some individuals cannot receive the vaccine.

The herd immunity threshold ($H$) required to interrupt community transmission of a pathogen is calculated as $H = 1 - 1/R_0$, where $R_0$ is the basic reproduction number. For that threshold to be reached through vaccination, vaccination coverage ($V_c$) must satisfy the condition $V_c \times E \ge H$, where $E$ is the vaccine effectiveness on a decimal scale.

In a community, a measles outbreak is detected with $R_0 = 15$. The available vaccine has an estimated effectiveness of $95\ \%$ ($E = 0,95$).

1. a) Calculate the minimum percentage of immune individuals ($H$) that the population requires to stop the outbreak and determine the minimum vaccination coverage ($V_c$) necessary to achieve it.
2. b) Explain the immunological basis of the herd effect: why is a newborn who cannot yet receive the vaccine protected if that coverage is reached?
3. c) If in that community vaccination coverage falls to $88\ \%$, predict what will happen to the transmission of the virus. Furthermore, justify why a disease like influenza ($R_0 \approx 1,5$) tolerates much lower vaccination coverages than measles without causing uncontrolled epidemics.

In the serological monitoring of an individual vaccinated against tetanus, it is observed that 10 years after their last dose, circulating antibodies (IgG) have fallen below the protective threshold (0,01 IU/mL). However, after administering a booster dose, they reach highly protective concentrations (> 1,0 IU/mL) in just 4 days, compared to the 12 days it took to generate them after their first vaccination in childhood.

1. a) Interpret the temporal and quantitative difference observed after the booster dose, identifying the cell populations and the selection and differentiation processes responsible.
2. b) If circulating antibodies were undetectable at 10 years, justify why it cannot be stated that the individual had completely lost their active immunity.
3. c) Predict what would happen to the response to the booster dose if the patient had developed an acquired immunodeficiency that selectively destroyed their helper T lymphocytes ($T_H$).

The yellow fever vaccine contains live attenuated viruses and protects for decades with a single dose, while the flu vaccine (inactivated viruses) must be administered every year. Reason why vaccines of live microorganisms usually generate longer-lasting immunity and why the composition of the flu vaccine changes every season.

In a preclinical trial using an animal model to stop a highly variable retrovirus, a vaccine based on the viral envelope protein isolated from a specific strain (strain A) is inoculated. The serum of the vaccinated animals successfully neutralises strain A in vitro, but it is completely ineffective against samples of the same virus obtained from the reservoir eight months later.

1. a) Interpret this failure at a genetic and molecular level: what changes have occurred in the virus and why have the neutralizing antibodies and the receptors of pre-existing lymphocytes ceased to be useful?
2. b) If you wanted to design a universal vaccine effective against all variants of that virus, propose what type of regions of the viral proteins the antigenic target should focus on and biologically justify your proposal.
3. c) As an alternative therapy, an intravenous infusion of broadly neutralizing monoclonal antibodies is administered to a patient. Classify the type of immunity provided and reasonably predict whether the patient will remain protected in the long term against future reinfections.