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Biochemistry of the Immune System: Antibodies, Antigens and Cytokines

February 22, 2025

Biochemistry of the Immune System: Antibodies, Antigens and Cytokines – Molecular Interactions and Functions

Introduction

The immune system is a highly specialized network of cells and molecules that protect the body from pathogens and foreign substances. At the molecular level, the key players include antibodies, antigens, and cytokines. Understanding their biochemical properties and interactions is crucial for immunology, medicine, and biotechnology applications.

Role of cytokines in immunity,
How antibodies fight infections,
Antigens and immune response explained,
Biochemistry of immune signaling,
Immunoglobulins and their functions.

1. Antigens: Nature and Biochemical Properties

1.1 What Are Antigens?

Antigens are molecules that trigger an immune response by interacting with immune system components like antibodies and T-cell receptors.

1.2 Types of Antigens

Exogenous Antigens: Originating from external pathogens (e.g., bacteria, viruses, and fungi).
Endogenous Antigens: Produced within infected or altered cells (e.g., viral proteins in infected cells, cancer antigens).
Autoantigens: Normal cellular components mistakenly targeted in autoimmune diseases.
Neoantigens: New antigens arising from mutations, often found in tumors.

1.3 Structural Characteristics of Antigens

Proteins (e.g., viral coat proteins, bacterial exotoxins)
Polysaccharides (e.g., bacterial capsules)
Lipids and nucleic acids (less commonly antigenic but can be immunogenic when bound to proteins)

2. Antibodies: Structure and Functions

2.1 What Are Antibodies?

Antibodies (Immunoglobulins, Ig) are Y-shaped glycoproteins produced by B cells to neutralize antigens.

2.2 Classes of Antibodies

IgG: Most abundant; provides long-term immunity.
IgA: Found in mucosal secretions (saliva, tears, breast milk).
IgM: First antibody produced in response to infection.
IgE: Involved in allergic responses and parasitic infections.
IgD: Functions in B cell activation.

2.3 Structure of an Antibody

Variable region: Binds specifically to an antigen.
Constant region: Determines the antibody’s class and effector functions.
Light and heavy chains: Form antigen-binding sites.

2.4 Functions of Antibodies

Neutralization: Blocks pathogens from infecting cells.
Opsonization: Marks pathogens for destruction by phagocytes.
Complement activation: Triggers the immune cascade leading to pathogen lysis.
Antibody-dependent cellular cytotoxicity (ADCC): Directs immune cells to kill infected cells.

3. Cytokines: Molecular Messengers of Immunity

3.1 What Are Cytokines?

Cytokines are small proteins that regulate immune responses, inflammation, and hematopoiesis.

3.2 Types of Cytokines and Their Functions

Pro-inflammatory Cytokines (Stimulate Immune Response)

Interleukin-1 (IL-1): Promotes fever and inflammation.
Tumor Necrosis Factor-alpha (TNF-α): Activates inflammatory pathways and cell death.
Interleukin-6 (IL-6): Stimulates acute phase response and B cell differentiation.

Anti-inflammatory Cytokines (Suppress Immune Response)

Interleukin-10 (IL-10): Inhibits inflammation and regulates immune balance.
Transforming Growth Factor-beta (TGF-β): Controls cell differentiation and tissue repair.

Cytokines in Immune Cell Communication

Interferons (IFNs): Antiviral responses and activation of immune cells.
Interleukin-2 (IL-2): T cell proliferation and survival.
Colony-Stimulating Factors (CSFs): Stimulate bone marrow to produce immune cells.

3.3 Cytokine Storm and Disease Implications

Excessive cytokine release (cytokine storm) can cause severe inflammation and tissue damage, seen in COVID-19, sepsis, and autoimmune diseases.

4. Biochemical Interactions Between Antigens, Antibodies, and Cytokines

4.1 How Antibodies Bind to Antigens

Lock-and-key model: Antibodies recognize specific epitopes on antigens.
Affinity and Avidity: Determines the strength of antibody-antigen interactions.
Cross-reactivity: Some antibodies recognize multiple related antigens.

4.2 Role of Cytokines in Immune Responses

Cytokines influence B and T cell activation, differentiation, and function.
Regulation of inflammation and immune memory formation.

4.3 Immunological Memory and Vaccine Development

Memory B and T cells ensure faster and stronger responses upon re-exposure to pathogens.
mRNA vaccines (e.g., COVID-19 vaccines) use antigen encoding to elicit protective immune responses.

5. Clinical and Biotechnological Applications

5.1 Antibody-Based Therapies

Monoclonal antibodies: Used for cancer immunotherapy (e.g., Rituximab, Pembrolizumab).
Immunoglobulin therapy: Passive immunity against infections.
Allergy treatments: Anti-IgE therapy (Omalizumab).

5.2 Cytokines in Disease Treatment

Interferons for viral infections and cancer.
Interleukins for boosting immune responses in immunodeficiency.

5.3 Diagnostic Applications

ELISA (Enzyme-linked immunosorbent assay): Detects antigen-antibody interactions.
Western Blot: Identifies proteins using specific antibodies.
Flow Cytometry: Analyzes immune cell populations.

6. Conclusion

Understanding the biochemistry of antigens, antibodies, and cytokines is crucial for immunology, vaccine development, and medical advancements. The intricate biochemical pathways governing immune responses hold potential for treating infectious diseases, autoimmune conditions, and cancer.

Relevant Website Links

National Center for Biotechnology Information (NCBI): https://www.ncbi.nlm.nih.gov
World Health Organization (WHO) – Immunization: https://www.who.int/immunization
Nature Immunology: https://www.nature.com/ni

MCQs on Biochemistry of the Immune System: Antibodies, Antigens and Cytokines

1. Which of the following best describes an antigen?

A) A type of immune cell
B) A foreign substance that induces an immune response
C) A molecule that suppresses immune activity
D) A type of antibody

Answer: B
Explanation: An antigen is a foreign molecule, typically a protein or polysaccharide, that triggers an immune response by binding to a specific antibody or T-cell receptor.

2. What is the basic structural unit of an antibody?

A) Single polypeptide chain
B) Two heavy chains and two light chains
C) Three heavy chains and one light chain
D) Lipid bilayer

Answer: B
Explanation: An antibody (immunoglobulin) consists of two identical heavy chains and two identical light chains linked by disulfide bonds, forming a Y-shaped structure.

3. Which immunoglobulin is most abundant in the bloodstream?

A) IgA
B) IgD
C) IgE
D) IgG

Answer: D
Explanation: IgG is the most abundant antibody in the serum and provides long-term immunity by neutralizing pathogens and facilitating their clearance.

4. The part of an antigen that is recognized by an antibody is called:

A) Epitope
B) Paratope
C) Hapten
D) Isotope

Answer: A
Explanation: The epitope is the specific region on an antigen that is recognized and bound by an antibody or a T-cell receptor.

5. Which of the following cytokines is primarily involved in stimulating fever?

A) Interleukin-10 (IL-10)
B) Tumor Necrosis Factor-alpha (TNF-α)
C) Interleukin-1 (IL-1)
D) Interferon-beta (IFN-β)

Answer: C
Explanation: IL-1 is a pro-inflammatory cytokine that induces fever by acting on the hypothalamus to increase body temperature.

6. What is the function of cytokines in the immune system?

A) Directly kill pathogens
B) Enhance the activity of immune cells
C) Suppress the immune system completely
D) Act as structural proteins in antibodies

Answer: B
Explanation: Cytokines are signaling proteins that regulate immunity by stimulating immune cells, promoting inflammation, or modulating immune responses.

7. Which immunoglobulin is primarily responsible for allergic reactions?

A) IgG
B) IgA
C) IgM
D) IgE

Answer: D
Explanation: IgE binds to mast cells and basophils, triggering the release of histamine and other mediators involved in allergic responses.

8. Which antigen-presenting cells (APCs) are the most effective at initiating an immune response?

A) Macrophages
B) Dendritic cells
C) B cells
D) Neutrophils

Answer: B
Explanation: Dendritic cells are the most potent antigen-presenting cells (APCs), capable of activating naive T cells and initiating an adaptive immune response.

9. The first antibody produced during an initial immune response is:

A) IgA
B) IgE
C) IgM
D) IgG

Answer: C
Explanation: IgM is the first antibody produced in response to an infection and is highly effective in forming antigen-antibody complexes.

10. What type of immunity do antibodies provide?

A) Innate immunity
B) Passive immunity
C) Cell-mediated immunity
D) Humoral immunity

Answer: D
Explanation: Antibodies are part of humoral immunity, which involves the production of specific proteins by B cells to neutralize pathogens.

11. Which class of cytokines is primarily involved in antiviral responses?

A) Interleukins
B) Tumor necrosis factors
C) Interferons
D) Chemokines

Answer: C
Explanation: Interferons (IFNs) are cytokines that help fight viral infections by enhancing the antiviral state of cells and activating immune cells.

12. What is the primary function of IgA antibodies?

A) Complement activation
B) Mucosal immunity
C) Opsonization
D) Antibody-dependent cell-mediated cytotoxicity (ADCC)

Answer: B
Explanation: IgA is mainly found in mucosal secretions like saliva, tears, and breast milk, providing protection against mucosal pathogens.

13. Which cells are responsible for producing antibodies?

A) Macrophages
B) T cells
C) B cells
D) Neutrophils

Answer: C
Explanation: B cells differentiate into plasma cells, which produce antibodies in response to antigens.

14. What is a hapten?

A) A complete antigen
B) A small molecule that requires a carrier protein to elicit an immune response
C) A type of antibody
D) A cytokine

Answer: B
Explanation: Haptens are small molecules that can bind to antibodies but are not immunogenic unless attached to a carrier protein.

15. Which type of T cell directly kills infected cells?

A) Helper T cells
B) Regulatory T cells
C) Cytotoxic T cells
D) Memory B cells

Answer: C
Explanation: Cytotoxic T cells (CD8+ T cells) recognize and kill virus-infected and cancerous cells by inducing apoptosis.

16. The complement system enhances immune defense by:

A) Producing antibodies
B) Promoting phagocytosis and cell lysis
C) Suppressing inflammation
D) Inhibiting cytokine release

Answer: B
Explanation: The complement system enhances immune defense through opsonization, membrane attack complex (MAC) formation, and inflammation.

17. The major histocompatibility complex (MHC) is involved in:

A) Cytokine production
B) Antigen presentation
C) Antibody synthesis
D) B cell maturation

Answer: B
Explanation: MHC molecules present antigens to T cells, helping to initiate adaptive immune responses.

18. Which cytokine is a key mediator of inflammation?

A) Interleukin-2
B) Tumor Necrosis Factor-alpha (TNF-α)
C) Interferon-gamma
D) Interleukin-4

Answer: B
Explanation: TNF-α is a pro-inflammatory cytokine that plays a key role in immune responses and inflammatory processes.

19. What type of immunity is provided by a vaccine?

A) Innate immunity
B) Artificial active immunity
C) Natural passive immunity
D) Artificial passive immunity

Answer: B
Explanation: Vaccines stimulate the immune system to generate memory cells, providing artificial active immunity.

20. Which of the following immunoglobulins can cross the placenta?

A) IgA
B) IgD
C) IgE
D) IgG

Answer: D
Explanation: IgG is the only antibody that can cross the placenta, providing passive immunity to the fetus.

21. What is the primary function of memory B cells?

A) Produce cytokines
B) Present antigens to T cells
C) Provide long-term immunity by recognizing previously encountered antigens
D) Directly kill infected cells

Answer: C
Explanation: Memory B cells remember past infections and rapidly produce antibodies upon re-exposure to the same antigen.

22. Which immunoglobulin is the first line of defense in the respiratory and gastrointestinal tracts?

A) IgA
B) IgG
C) IgE
D) IgM

Answer: A
Explanation: IgA is present in mucosal secretions (e.g., saliva, tears, and gut lining) and protects against pathogens at mucosal surfaces.

23. Which cells are primarily responsible for antigen presentation to T cells?

A) Neutrophils
B) Dendritic cells
C) Mast cells
D) Natural killer (NK) cells

Answer: B
Explanation: Dendritic cells are professional antigen-presenting cells (APCs) that present antigens to naive T cells and initiate immune responses.

24. Which cytokine promotes the differentiation of naive T cells into Th1 cells?

A) Interleukin-4 (IL-4)
B) Interferon-gamma (IFN-γ)
C) Tumor Necrosis Factor-beta (TNF-β)
D) Interleukin-10 (IL-10)

Answer: B
Explanation: IFN-γ is a key cytokine that promotes the differentiation of naive T cells into Th1 cells, which enhance cell-mediated immunity.

25. Which immunoglobulin is primarily found in breast milk?

A) IgM
B) IgE
C) IgA
D) IgD

Answer: C
Explanation: IgA is secreted in breast milk and provides passive immunity to newborns by protecting mucosal surfaces.

26. What is the function of regulatory T cells (Tregs)?

A) Activate B cells
B) Suppress immune responses to prevent autoimmunity
C) Directly kill infected cells
D) Produce large amounts of antibodies

Answer: B
Explanation: Regulatory T cells (Tregs) suppress immune responses to maintain immune tolerance and prevent autoimmune diseases.

27. Which cytokine is critical for the survival and proliferation of T cells?

A) Interleukin-2 (IL-2)
B) Interferon-alpha (IFN-α)
C) Tumor Necrosis Factor-alpha (TNF-α)
D) Interleukin-6 (IL-6)

Answer: A
Explanation: IL-2 is essential for T-cell proliferation and survival, playing a critical role in immune responses.

28. Which immune response is mediated by T cells rather than antibodies?

A) Humoral immunity
B) Passive immunity
C) Cell-mediated immunity
D) Innate immunity

Answer: C
Explanation: Cell-mediated immunity is driven by T cells, which help eliminate intracellular pathogens and infected cells.

29. Which of the following is NOT a characteristic of innate immunity?

A) Rapid response
B) No memory formation
C) Highly specific antigen recognition
D) Involvement of natural killer (NK) cells

Answer: C
Explanation: Innate immunity is non-specific and does not recognize specific antigens, unlike adaptive immunity, which is highly specific.

30. Which of the following is an example of passive immunity?

A) Recovery from a viral infection
B) Vaccination against a disease
C) Transfer of maternal antibodies to a newborn
D) Activation of T cells after infection

Answer: C
Explanation: Passive immunity occurs when antibodies are transferred from another individual, such as maternal antibodies passed to a newborn through the placenta or breast milk.