Clinical Cases of Immunity

Subsection: Immunology

Case Scenario 1:

A 25-year-old man presents to the clinic with complaints of high-grade fever, malaise, and sore throat for the past 3 days. On examination, he has enlarged cervical lymph nodes and a positive rapid antigen test for Streptococcus pyogenes. His medical history reveals he had a similar infection a year ago but recovered without complications.

1. a) Based on the case, describe the role of both innate and acquired immunity in combating the infection.
2. b) Explain the immunological basis of memory response and how it applies to this patient.
3. c) Classify immunity and identify which types are involved in this case, with reasoning.

Answer:

1. a) Role of Innate and Acquired Immunity in Combating Infection:

In this case of Streptococcal pharyngitis, both innate and acquired immunity play crucial roles:

Innate Immunity:

• First line of defense: Includes physical barriers like the mucous membranes of the oropharynx and chemical agents like lysozyme in saliva.
• Phagocytic cells (e.g., neutrophils and macrophages) recognize pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors (PRRs).
• Cytokines (e.g., IL-1, TNF-α) are released to induce inflammation.
• Complement system is activated to opsonize bacteria and recruit immune cells.

Acquired Immunity:

• Antigen-presenting cells (APCs) like dendritic cells present streptococcal antigens to naive T cells.
• Helper T cells (CD4+) activate B cells, which differentiate into plasma cells and produce antibodies specific to streptococcal antigens.
• Cytotoxic T cells (CD8+) may assist if intracellular pathogens are present.
• Antibodies aid in neutralization, opsonization, and complement activation.

 

1. b) Immunological Basis of Memory Response:

When the patient had a previous Streptococcus pyogenes infection:

• Primary immune response was generated.
• Some B and T cells differentiated into memory cells.
• Upon re-exposure (current infection), the secondary immune response is:
  • Faster (due to pre-formed memory cells),
  • Stronger (higher antibody titer),
  • More specific and efficient.

This explains why the current infection may resolve more rapidly or with fewer complications compared to the primary episode.

1. c) Classification of Immunity and Types Involved in This Case:

Classification:

1. Innate Immunity
   • Non-specific, present from birth, includes barriers and phagocytes.
2. Acquired Immunity
   • Active Immunity:
     • Natural: Through infection (as in this case).
     • Artificial: Through vaccination.
   • Passive Immunity:
     • Natural: Maternal antibodies.
     • Artificial: Injected antibodies (e.g., antitoxins).

Types Involved in This Case:

• Innate immunity: Immediate response to the bacterial invasion.
• Natural active acquired immunity:
  • From previous exposure and memory cell formation.
  • The current immune response is enhanced by immunological memory.

 

Case Scenario 2:

A 2-year-old child is brought to the emergency department after a stray dog bite. The child is given anti-rabies immunoglobulin (RIG) along with the first dose of rabies vaccine. The parents ask the doctor why both are needed and whether these injections will prevent the disease.

1. a) What are immunoglobulins and what role do they play in immunity?
2. b) List the different types of immunoglobulins (Ig classes) and describe their major functions.

Answer:

1. a) What Are Immunoglobulins and Their Role in Immunity:

Immunoglobulins (Ig) are glycoprotein molecules produced by plasma cells (activated B lymphocytes) in response to antigens. They function as antibodies, which are essential components of the humoral immune response.

• Each immunoglobulin has a variable region (for antigen binding) and a constant region (which determines its class and function).
• They neutralize pathogens, opsonize microbes for phagocytosis, and activate the complement system.

In the given case, anti-rabies immunoglobulin (RIG) provides immediate passive immunity by neutralizing the virus at the site of exposure, preventing its spread to the central nervous system.

 

1. b) Classes of Immunoglobulins and Their Functions:

There are five major classes of immunoglobulins:

Class	Main Location	Key Functions
IgG	Blood, tissues, placenta	Most abundant; crosses placenta; activates complement; long-term immunity
IgA	Mucosal secretions (saliva, tears, breast milk, GI tract)	Mucosal defense; prevents microbial adherence
IgM	Blood and lymph (first responder)	First antibody in primary response; strong complement activation
IgE	Bound to mast cells and basophils	Mediates allergic reactions and defense against parasites
IgD	Surface of immature B cells	Unclear function; may help in B cell activation

 

Subsection: Vaccines

Case Scenario 3:

A 6-month-old infant is brought to the pediatric clinic for routine immunization. The mother expresses concern about the number of vaccines the baby has already received and asks whether these many vaccines are truly necessary. The child’s vaccination card shows that BCG, OPV, Hepatitis B, and DPT have already been administered.

1. a) Explain how vaccines work to provide immunity in infants.
2. b) Classify the different types of vaccines and provide examples relevant to the child’s vaccination schedule.
3. c) What is herd immunity? How does vaccination contribute to herd immunity in the community?

Answer:

1. a) How Vaccines Provide Immunity in Infants:

Vaccines stimulate the immune system to recognize and fight specific pathogens without causing the disease.

• They contain antigenic components (e.g., killed or weakened pathogens, or parts like proteins/toxoids).
• When introduced into the body, vaccines activate the acquired immune system:
  • B lymphocytes produce specific antibodies.
  • T lymphocytes help in cellular response and memory formation.
• Memory cells are generated, which enable a faster and stronger response upon future exposure to the actual pathogen.
• In infants, vaccines help build immunity early in life when their immune systems are still developing.

1. b) Classification of Vaccines with Relevant Examples:

Vaccines are classified based on how the antigen is prepared:

Type of Vaccine	Description	Examples (from schedule)
Live attenuated vaccines	Contain weakened forms of the pathogen	BCG, OPV (Oral Polio Vaccine)
Inactivated (killed)	Contain killed organisms	IPV (Inactivated Polio Vaccine)
Toxoid vaccines	Contain inactivated bacterial toxins	DPT (contains diphtheria & tetanus toxoids)
Subunit/Conjugate vaccines	Contain parts of the pathogen (e.g., proteins)	Hepatitis B (recombinant HBsAg)

These vaccines aim to mimic natural infection, prompting the immune system to mount a protective response.

1. c) Herd Immunity and Role of Vaccination:

Herd immunity occurs when a large portion of a population becomes immune to a disease, making its spread unlikely, even among unvaccinated individuals.

• It protects vulnerable groups who cannot be vaccinated (e.g., infants, immunocompromised individuals).
• For herd immunity to be effective, a high vaccination coverage (usually >80–90%) is essential.

Vaccination contributes by:

• Reducing the number of susceptible hosts.
• Interrupting transmission chains.
• Controlling outbreaks and contributing to disease eradication (e.g., smallpox)

 

Case Scenario 4:

A 9-month-old infant is brought to the primary health center for the first dose of the measles vaccine, which is a live attenuated vaccine. The mother expresses concern, asking how a “live” virus can be safe for her baby and why it is preferred in some cases over other types of vaccines.

1. a) What are live attenuated vaccines, and how do they stimulate immunity?
2. b) Discuss the advantages and disadvantages of live attenuated vaccines.
3. c) List important live attenuated vaccines used in the national immunization schedule and state any contraindications for their use.

Answer:

1. a) What are Live Attenuated Vaccines and How They Stimulate Immunity:

Live attenuated vaccines contain weakened (attenuated) forms of the actual disease-causing organisms. These organisms are modified in the laboratory so they cannot cause disease in healthy individuals but can still replicate to a limited extent.

• When administered, they mimic a natural infection closely.
• This leads to a strong activation of the immune system, involving:
  • Humoral immunity (B cells → antibodies),
  • Cell-mediated immunity (T cells),
  • And long-lasting immunological memory.
• A single dose often provides long-term protection due to strong immune stimulation.

In the case of the measles vaccine, the attenuated virus replicates in the host without causing illness, triggering a protective immune response.

 

1. b) Advantages and Disadvantages of Live Attenuated Vaccines:

Advantages:

• Elicit strong, long-lasting immunity (both humoral and cellular).
• Often require fewer doses or boosters.
• Closely simulate a natural infection, producing a more effective response.

Disadvantages:

• Risk in immunocompromised individuals – even attenuated organisms can cause disease.
• Require careful storage (cold chain maintenance).
• Potential for reversion to virulence (rare).
• Not suitable during pregnancy.

 

1. c) Important Live Attenuated Vaccines in National Immunization Schedule & Contraindications:

Examples of Live Attenuated Vaccines:

Vaccine	Target Disease
BCG	Tuberculosis
OPV	Poliomyelitis
Measles	Measles
MMR	Measles, Mumps, Rubella
JE (SA-14-14-2)	Japanese Encephalitis (in some states)

Contraindications:

• Severe immunodeficiency (e.g., HIV/AIDS, cancer chemotherapy).
• Pregnancy (due to potential harm to fetus).
• Severe allergic reaction to vaccine components.
• Acute febrile illness (vaccine should be postponed).

 

Case Scenario 5:

A 2-month-old infant is brought to the immunization clinic for routine vaccination. The child receives the Inactivated Polio Vaccine (IPV) as part of the national immunization schedule. The mother asks how this vaccine is different from the oral polio vaccine (OPV) that her older child received.

1. a) What are killed (inactivated) vaccines, and how do they induce immunity?
2. b) Compare killed vaccines with live attenuated vaccines in terms of immune response, safety, and use.
3. c) List commonly used killed vaccines in the national immunization schedule and mention their major advantages and limitations.

Answer:

1. a) What Are Killed (Inactivated) Vaccines and How They Induce Immunity:

Killed (inactivated) vaccines are prepared by killing or inactivating the pathogen (usually using heat, chemicals, or radiation) so that it cannot replicate or cause disease, but retains its antigenic properties.

• When administered, these antigens are recognized by the immune system, primarily stimulating humoral immunity (antibody production by B cells).
• These vaccines generally do not replicate in the host, hence multiple doses and boosters are often required to maintain long-term immunity.
• Example: Inactivated Polio Vaccine (IPV) introduces inactivated poliovirus strains that cannot cause paralysis but still prompt protective immunity.

1. b) Comparison: Killed vs. Live Attenuated Vaccines

Feature	Killed (Inactivated) Vaccines	Live Attenuated Vaccines
Pathogen replication	No	Yes (limited replication)
Type of immunity	Mainly humoral	Both humoral and cell-mediated
Number of doses	Requires multiple doses and boosters	Often effective with a single or fewer doses
Safety	Safer in immunocompromised and pregnant patients	Contraindicated in immunocompromised & pregnancy
Cold chain sensitivity	Less sensitive	Highly sensitive
Examples	IPV, Hepatitis A, Rabies	OPV, BCG, MMR, Measles

 

1. c) Common Killed Vaccines in National Immunization Schedule & Their Advantages/Limitations:

Examples of Killed Vaccines:

Vaccine	Disease Prevented
IPV	Poliomyelitis
Hepatitis A vaccine	Hepatitis A
Rabies vaccine	Rabies
Influenza (injectable)	Seasonal flu
Whole-cell pertussis (wP) in DPT	Pertussis
JE (inactivated) vaccine	Japanese Encephalitis (in some regions)

Advantages:

• Cannot revert to virulence (since pathogens are dead).
• Safe in immunocompromised individuals and during pregnancy.
• Stable for storage and transport, less dependent on strict cold chain.

Limitations:

• Weaker immune response than live vaccines.
• Primarily antibody-mediated immunity, little or no cell-mediated response.
• Require multiple doses and boosters to maintain protection.

 

Case Scenario 6:

A 6-week-old infant is brought to the primary health center for the first dose of the DPT (Diphtheria, Pertussis, and Tetanus) vaccine as part of the national immunization schedule. The medical officer explains to the mother that diphtheria and tetanus components of the vaccine are toxoid vaccines.

1. a) What are toxoid vaccines, and how do they stimulate immunity?
2. b) Explain the importance of booster doses for toxoid vaccines and the concept of immunological memory.
3. c) List examples of toxoid vaccines in use and describe their advantages and limitations.

 

1. a) What Are Toxoid Vaccines and How Do They Stimulate Immunity:

Toxoid vaccines are made by inactivating bacterial exotoxins (using formalin or heat), converting them into non-toxic but immunogenic forms called toxoids.

• They do not contain live organisms, but the inactivated toxin still retains the structure necessary to stimulate the immune response.
• The immune system produces antibodies (antitoxins) specifically targeting the toxin produced by the bacteria.
• B cells recognize the toxoid and generate antibody-mediated immunity and memory B cells, providing long-term protection.

Examples in DPT:

• Diphtheria toxoid protects against the diphtheria toxin.
• Tetanus toxoid protects against tetanospasmin.

 

1. b) Importance of Booster Doses and Immunological Memory:

• Toxoid vaccines stimulate mainly humoral (antibody-mediated) immunity.
• Initial doses generate primary immune response, but over time, antibody levels decline.
• Booster doses re-expose the immune system to the antigen, leading to a secondary immune response that is:
  • Faster,
  • Stronger,
  • Longer-lasting.

This reinforces immunological memory, ensuring continued protection, especially against tetanus, which has no natural immunity and can occur without prior warning.

Example: Tetanus boosters are recommended every 10 years.

 

1. c) Examples of Toxoid Vaccines and Their Advantages & Limitations:

Examples:

Toxoid Vaccine	Disease Prevented
Diphtheria toxoid	Diphtheria
Tetanus toxoid	Tetanus
Combined (DT, DPT, Td)	Multiple diseases

Advantages:

• Safe and stable, cannot revert to toxic or infectious form.
• Can be used in immunocompromised individuals and pregnant women.
• Part of combined vaccines (e.g., DPT), improving coverage.

Limitations:

• Do not prevent infection, only neutralize the toxin.
• Multiple doses and boosters needed to maintain immunity.
• Primarily antibody-mediated; no cellular immunity.