IM [6.3.1] Introduction to the immune system 2024. 2025

Introduction to the Immune System

This session aims to provide foundational knowledge about the immune system, covering aspects from basic concepts to cell differentiation and immune responses. The immune system is a dynamic network of cells, tissues, and organs that work together to defend the body against harmful pathogens, including viruses, bacteria, fungi, and parasites.

Learning Objectives

By the end of the session, you should be able to:

• Describe the differences between innate and adaptive immunity in terms of response time, specificity, and memory.

• Identify the main types of cells involved in the immune response and their specific functions.

• Explain the complete development of immune cells from hematopoietic stem cells, including molecular signals involved.

• List and elaborate on the functions of various immune cells, including their role in homeostasis and tissue repair.

Functions of the Immune System

The immune system's primary function is to combat infections caused by diverse pathogens, categorized as follows:

• Viruses: Intracellular parasites that require a host cell for replication.

• Bacteria: Single-celled organisms that can be either harmful or beneficial to the host.

• Fungi: Multi-cellular organisms that can cause infections, especially in immunocompromised individuals.

• Parasites: Organisms that live on or in a host and derive nourishment at the host's expense.

Types of Immunity

Arm of Immunity

The immune system is divided into two main arms:

1. Innate Immunity: The body's first line of defense, providing immediate but non-specific responses to a wide range of pathogens. Key components include physical barriers (like the skin), chemical barriers (like saliva and gastric acid), and cellular components (like phagocytes).

2. Adaptive (Acquired) Immunity: A more sophisticated second line of defense that develops over time, providing specific responses to pathogens identified by unique antigens. It has a memory component that allows for quicker and more efficient responses upon re-exposure to the same pathogen.

Differences Between Innate and Adaptive Immunity

Cells of the Immune System

White blood cells (WBCs), also known as leukocytes, are critical components of the immune system and originate from hematopoietic stem cells in the bone marrow. They migrate to various tissues to monitor, recognize, and react against infections and foreign bodies.

Development of Immune Cells

Cell Differentiation

The differentiation path includes:

• Hematopoietic Stem Cell: The origin point for all immune cells, capable of self-renewal and differentiation.

• Lymphoid Stem Cell: Differentiate into:

  • T cells: Key players in adaptive immunity, crucial for cell-mediated responses.

  • B cells: Responsible for producing antibodies that neutralize pathogens.

  • Natural killer (NK) cells: Important in early responses to viral infections and tumor surveillance.

• Myeloid Progenitor: Differentiates into:

  • Granulocytes (e.g., neutrophils, eosinophils, basophils): Essential for innate immune response.

  • Monocytes/Macrophages: Engulf and digest pathogens and debris.

Function of Immune Cells

Types of Immune Cells

• Granulocytes: Include multiple cell types crucial for innate immunity:

  • Neutrophils: The most abundant WBCs (40-80% of leukocytes), short-lived, and primarily protect against bacterial infections, contributing to pus formation upon death.

  • Eosinophils: Constitute 1-6% of WBCs and play a vital role in defending against parasitic infections and allergic reactions.

  • Basophils: Comprise 0-1% of WBCs, involved in inflammatory responses and play roles in allergic reactions.

• Monocytes/Macrophages: Make up 2-10% of WBCs. Monocytes circulate in the blood and transform into macrophages upon migration into tissues, where they perform phagocytosis, engulfing pathogens and cellular debris.

• Lymphocytes: Account for 20-40% of WBCs. They are categorized into two main types:

  • T-Lymphocytes: Mature in the thymus and are further categorized into:

    • T-helper Cells: Activate macrophages and B cells, enhancing the adaptive immune response.

    • Cytotoxic T Cells: Target and destroy infected cells.

  • B-Lymphocytes: Mature in the bone marrow and produce antibodies through differentiation into plasma cells, aiding in humoral immunity.

• Natural Killer Cells: A unique population of lymphocytes that attack abnormal host cells, particularly virally infected or tumor cells.

Mechanisms of Innate Immunity

1. Mechanical Barriers: Intact skin and mucous membranes serve as physical barriers, preventing pathogen entry.

2. Humoral Defense Mechanisms: Body fluids contain soluble substances like lysozymes that aim at destroying pathogens.

3. Cellular Defenses: Cells like macrophages and neutrophils perform phagocytosis, effectively engulfing and destroying foreign particles.

4. Normal Bacterial Flora: Compete with pathogens for resources and produce antimicrobial substances that inhibit pathogen growth.

Summary

Throughout the session, we've explored the complex and layered structure of the immune system, detailing the roles, differentiation, and functions of various immune cells and emphasizing their importance in maintaining health by effectively combating pathogens and contributing to tissue homeostasis.

Below are 15 case-based Multiple Choice Questions (SBA) related to the immune system, each with five choices.

Case 1

A 6-year-old child presents with recurrent bacterial infections, particularly of the ears and lungs. Laboratory tests reveal significantly low levels of immunoglobulin. Which of the following immune cells is most likely deficient in this child?

1. B cells

2. T cells

3. Natural killer cells

4. Neutrophils

5. Monocytes

Case 2

A patient with a viral infection has an increased number of cytotoxic T lymphocytes in their blood. What is the primary function of these cells?

1. To produce antibodies

2. To activate B cells

3. To directly kill infected cells

4. To enhance phagocytosis

5. To support macrophage activity

Case 3

A 35-year-old woman with a history of allergies presents with wheezing and hypotension after exposure to pollen. Which type of immune cell is primarily involved in the allergic response?

1. Neutrophils

2. Eosinophils

3. Basophils

4. T-helper cells

5. B cells

Case 4

During a study on innate immunity, which type of immune cell is identified as the first responder to sites of infection?

1. B cells

2. T cells

3. Neutrophils

4. Natural killer cells

5. Macrophages

Case 5

A 60-year-olds man is diagnosed with chronic myeloid leukemia. Which type of immune cell is primarily affected by this disease?

1. B cells

2. Myeloid progenitors

3. Lymphoid progenitors

4. Platelets

5. Eosinophils

Case 6

A patient is diagnosed with a virus that has caused a significant decrease in their CD4+ T cells. What is the most likely consequence of this decrease?

1. Reduced antibody production

2. Increased susceptibility to bacterial infections

3. Enhanced cytotoxic response

4. Decreased phagocytosis

5. Increased autoimmune responses

Case 7

A newly diagnosed patient with HIV presents with opportunistic infections. Which cell type is primarily involved in fighting off these infections?

1. B cells

2. T-helper cells

3. Natural killer cells

4. Eosinophils

5. Basophils

Case 8

A scientist is exploring the role of the complement system in immune response. Which of the following functions is NOT a role of the complement system?

1. Opsonization of pathogens

2. Lysis of foreign cells

3. Enhancing phagocytosis

4. Production of antibodies

5. Inflammatory response recruitment

Case 9

A 25-year-old woman presents with fatigue, recurrent infections, and weight gain. Laboratory tests indicate increased levels of IgM and decreased IgG. What immune condition might she be suffering from?

1. Severe Combined Immunodeficiency (SCID)

2. Common Variable Immunodeficiency (CVID)

3. Hyper-IgM syndrome

4. Chronic Granulomatous Disease (CGD)

5. Autoimmune Lymphoproliferative Syndrome (ALPS)

Case 10

A 45-year-old man has a severe allergic reaction after being stung by a bee. Which type of immune response is primarily responsible for this reaction?

1. Innate immunity

2. Humoral immunity

3. Cell-mediated immunity

4. Autoimmunity

5. Inflammatory response

Case 11

A child presents with symptoms of recurrent infections and is found to have a deficiency in antibody production. What is the most likely primary immune defect?

1. T-helper cell deficiency

2. B-cell deficiency

3. Neutrophil dysfunction

4. Phagocyte deficiency

5. Natural killer cell depletion

Case 12

A 30-year-old male receives a vaccination that stimulates an immune response. Which type of immunity does this vaccination primarily induce?

1. Passive immunity

2. Innate immunity

3. Active immunity

4. Humoral immunity

5. Cellular immunity

Case 13

A researcher is studying the role of dendritic cells in the immune response. Which of the following best describes the function of dendritic cells?

1. They produce antibodies

2. They present antigens to T cells

3. They kill infected cells directly

4. They secrete cytokines

5. They form the first line of defense against pathogens

Case 14

A patient is diagnosed with autoimmune hemolytic anemia. Which type of immune cell is likely producing antibodies that attack the patient’s red blood cells?

1. Neutrophils

2. CD4+ T cells

3. B cells

4. Natural killer cells

5. Eosinophils

Case 15

A young man with cystic fibrosis is being assessed for infections. Which immune cell plays a key role in clearing pulmonary bacterial infections?

1. B cells

2. Neutrophils

3. T cells

4. Dendritic cells

5. Monocytes

Here are the answers to the case-based Multiple Choice Questions:

Case 1

Answer: 1. B cellsExplanation: Low levels of immunoglobulin indicate a deficiency in B cells, which produce antibodies.

Case 2

Answer: 3. To directly kill infected cellsExplanation: Cytotoxic T lymphocytes primarily function to kill infected cells directly.

Case 3

Answer: 3. BasophilsExplanation: Basophils are primarily involved in allergic responses and release histamines upon activation.

Case 4

Answer: 3. NeutrophilsExplanation: Neutrophils are the first responders to sites of infection in the innate immune response.

Case 5

Answer: 2. Myeloid progenitorsExplanation: Chronic myeloid leukemia primarily affects myeloid progenitors.

Case 6

Answer: 1. Reduced antibody productionExplanation: A significant decrease in CD4+ T cells leads to reduced help for B cells, thus reducing antibody production.

Case 7

Answer: 2. T-helper cellsExplanation: T-helper cells are crucial for fighting off opportunistic infections in individuals with HIV.

Case 8

Answer: 4. Production of antibodiesExplanation: The complement system does not produce antibodies; that is the role of B cells.

Case 9

Answer: 3. Hyper-IgM syndromeExplanation: This condition is characterized by elevated IgM levels and decreased IgG, indicating a defect in class switching in B cells.

Case 10

Answer: 2. Humoral immunityExplanation: The immediate allergic response is primarily mediated by the humoral immune system, which involves antibodies.

Case 11

Answer: 2. B-cell deficiencyExplanation: A deficiency in antibody production likely indicates a problem with B cells.

Case 12

Answer: 3. Active immunityExplanation: Vaccinations stimulate an active immune response by the body's immune system.

Case 13

Answer: 2. They present antigens to T cellsExplanation: Dendritic cells are best known for presenting antigens to T cells to initiate an immune response.

Case 14

Answer: 3. B cellsExplanation: B cells are responsible for producing antibodies that can attack the body’s own red blood cells in autoimmune hemolytic anemia.

Case 15

Answer: 2. NeutrophilsExplanation: Neutrophils play a key role in clearing bacterial infections in the lungs, especially in cases like cystic fibrosis.