Lesson 3.3: Innate and Adaptive Immunity

Introduction

In this lesson, students will explore the foundational concepts of innate and adaptive immunity. Understanding our immune system is crucial for recognizing how the body defends itself against pathogens, and this knowledge is a cornerstone for future studies in microbiology and pharmacology. The objectives of this lesson include:

• Understanding the cells, organs, and molecules involved in innate and adaptive immunity.
• Learning about antigen presentation, the Major Histocompatibility Complex (MHC), T-cell and B-cell responses, and antibody function.
• Exploring the roles of complement proteins, cytokines, and the principles of immunization.
• Describing the coordination between innate and adaptive responses.
• Explaining antigen presentation and lymphocyte activation at a mechanistic level.

H2: Cells and Organs of the Immune System

The immune system comprises various cells and organs that work together to protect the body against infections.

Cells of the Immune System

The primary cells involved in the immune response include:

1. White Blood Cells (Leukocytes): These cells are crucial for immune responses, and they can be further categorized into:

• Phagocytes: These include macrophages and neutrophils, which engulf and digest pathogens. For example, when a bacterium invades the body, neutrophils can rapidly migrate to the site of infection and consume the bacteria through a process called phagocytosis.
• Lymphocytes: These include T cells and B cells. T cells are responsible for cell-mediated immunity, while B cells are responsible for humoral immunity (antibody production).

1. Antigen-Presenting Cells (APCs): These cells (like dendritic cells and macrophages) play a key role in processing and presenting antigens to T cells. When a dendritic cell encounters pathogens, it processes the pathogen's proteins and presents peptide fragments on its surface using MHC molecules. This is vital for T cell activation.

Organs of the Immune System

Several organs are involved in the development and function of immune cells:

1. Bone Marrow: This is where all immune cells originate from hematopoietic stem cells. B cells mature in the bone marrow.
2. Thymus: T cells mature in the thymus before entering the peripheral circulation.
3. Lymph Nodes: These are distributed throughout the body and serve as sites for immune responses. They filter lymphatic fluid and house various immune cells.
4. Spleen: The spleen filters blood, removes old or damaged red blood cells, and helps initiate immune responses to blood-borne pathogens.

Summary

In summary, the cells and organs of the immune system work in unison to identify and eliminate pathogens. Understanding these components is critical as they form the basis for how the immune system operates.

H2: Antigen Presentation and Major Histocompatibility Complex (MHC)

Antigen presentation is a vital process in the adaptive immune response, allowing T cells to recognize and respond to specific antigens.

Major Histocompatibility Complex (MHC)

MHC molecules are surface proteins that present antigens to T cells. There are two classes of MHC:

• MHC Class I: Found on nearly all nucleated cells, these molecules present endogenous antigens (e.g., from viral proteins produced inside an infected cell) to CD8+ cytotoxic T cells.
• MHC Class II: Present on professional APCs (e.g., dendritic cells, macrophages, and B cells), these molecules present exogenous antigens (e.g., from engulfed pathogens) to CD4+ helper T cells.

Example of Antigen Presentation

Let's consider a scenario where a virus infects a cell. As the infected cell produces viral proteins, these proteins are processed and presented on the cell surface by MHC Class I molecules. A CD8+ cytotoxic T cell recognizes the viral peptide-MHC Class I complex, and upon activation, it can induce apoptosis (programmed cell death) in the infected cell.

Conversely, when a dendritic cell engulfs a bacterium, it can process and present peptides from bacterial proteins on MHC Class II molecules. This interaction with a CD4+ helper T cell leads to its activation, resulting in the help of B cells to produce antibodies or the activation of other immune cells.

H2: T-cell and B-cell Responses

The adaptive immune system is characterized by its ability to respond specifically to pathogens and to remember them for a more rapid response upon re-exposure.

T-cell Responses

T cells can be divided into several subsets:

1. CD4+ Helper T Cells: These cells help orchestrate the immune response by releasing cytokines that activate other immune cells. For example, they promote B cell differentiation into plasma cells, which secrete antibodies.
2. CD8+ Cytotoxic T Cells: These cells directly kill infected or cancerous cells.

Example of T-cell response: After the activation of a CD4+ T cell through antigen presentation by a dendritic cell, it secretes cytokines that enhance the activity of macrophages, boosting their ability to engulf and destroy pathogens. Additionally, it promotes B cell proliferation and differentiation into plasma cells.

B-cell Responses

B cells are primarily responsible for antibody production. Their activation also relies on T helper cells.

Example of B-cell activation: Upon recognizing a specific antigen and receiving help from a CD4+ T cell, a B cell can proliferate and differentiate into a plasma cell. Plasma cells secrete antibodies specific to the antigen, facilitating its neutralization and clearance.

B cells also have a memory function: some activated B cells persist as memory B cells, allowing for a quicker response if the same antigen is encountered again.

H2: Antibody Function

Antibodies (or immunoglobulins) are proteins produced by B cells that recognize and bind to specific antigens.

Types of Antibodies

1. IgM: The first antibody produced during an initial immune response. It effectively clumps pathogens together, facilitating their removal.
2. IgG: The most abundant antibody in the bloodstream and important for long-term immunity. It can cross the placenta and provide passive immunity to the fetus.
3. IgA: Found in mucosal areas (e.g., gut, respiratory tract) and secreted in saliva and breast milk, it plays a crucial role in mucosal immunity.
4. IgE: Involved in allergic reactions and responses to parasitic infections.
5. IgD: Functions primarily as a receptor on B cells that helps initiate their activation.

Mechanism of Action

Antibodies bind to antigens through their variable region, forming an antigen-antibody complex. This binding can neutralize the pathogen, block its entry into cells, opsonize it for phagocytosis, or activate the complement cascade to lyse the pathogen.

Example of antibody action: When IgG binds to a virus, it can block the virus's ability to enter a host cell, thereby preventing infection. Additionally, the bound virus can attract phagocytes to destroy it through opsonization.

H2: Complement System and Cytokines

The complement system is a group of proteins that enhance the ability of antibodies and phagocytes to clear pathogens.

Complement Activation

The complement can be activated via three pathways:

1. Classical Pathway: Triggered by antibody-antigen complexes.
2. Lectin Pathway: Triggered by the binding of lectin to carbohydrate components on pathogens.
3. Alternative Pathway: Triggered spontaneously on pathogen surfaces.

Functions of the Complement System

• Opsonization: Enhancing phagocytosis by coating pathogens.
• Lysis: Forming the membrane attack complex (MAC) that directly lyses pathogens.
• Inflammation: Releasing cytokines that recruit immune cells to the site of infection.

Cytokines are signaling molecules that mediate and regulate immunity, inflammation, and hematopoiesis. They include interleukins, chemokines, and interferons.

Example of cytokine function: When a macrophage recognizes a pathogen, it produces cytokines that promote inflammation and recruit other immune cells like neutrophils to the infected area.

H2: Principles of Immunization

Immunization aims to promote immunity to specific pathogens by exposing the body to a harmless form of the pathogen.

Types of Vaccines

1. Live Attenuated Vaccines: Contain weakened forms of the pathogen (e.g., measles vaccine).
2. Inactivated Vaccines: Contain killed pathogens (e.g., polio vaccine).
3. Subunit Vaccines: Include fragments of the pathogen (e.g., hepatitis B vaccine).
4. mRNA Vaccines: Teach cells to produce a protein that triggers an immune response (e.g., COVID-19 vaccines).

Mechanism of Immunization

The introduction of vaccine components promotes the development of memory B and T cells without causing disease. If the individual encounters the actual pathogen later, the immune system can mount a faster and more robust response.

Conclusion

In this lesson, students has learned about the essential components of the innate and adaptive immune systems, including the cells, organs, and molecules critical to immune responses. By examining antigen presentation, the function of T and B cells, the role of antibodies, and the complement system, as well as the principles of immunization, students has built a solid foundation that will be essential for understanding more complex immunological concepts and their applications in clinical settings.

Study Notes

• The innate immune system provides the first line of defense and includes physical barriers and immune cells such as phagocytes.
• Adaptive immunity is specific and involves T cells and B cells, which are activated through antigen presentation.
• MHC molecules are critical for the recognition of antigens by T cells.
• Antibodies produced by B cells neutralize pathogens and mark them for destruction.
• The complement system aids in the opsonization and destruction of pathogens.
• Cytokines regulate immunity and inflammation and are vital for communication among immune cells.
• Immunization effectively primes the immune system against specific pathogens by introducing harmless forms of those pathogens.