Immunology: Key Concepts, Antigens, and Immune Responses

Posted on Mar 27, 2025 in Biology

Q1: What is Immunology?

Immunology is the branch of biomedical science that studies the immune system, its components, functions, and how it protects the body from pathogens like bacteria, viruses, fungi, and parasites. It also deals with immune disorders, allergies, and immunotherapy.

Q2: Why is Innate Immunity the First Line of Defense?

Innate immunity provides immediate, non-specific protection against pathogens. It includes:

• Physical barriers: Skin, mucous membranes prevent pathogen entry.
• Chemical barriers: Enzymes (lysozyme in tears), acidic pH (stomach) destroy microbes.
• Cellular components: Phagocytes (macrophages, neutrophils) engulf and destroy pathogens.

Since it acts immediately, it serves as the first line of defense before adaptive immunity is activated.

Q3: What are Pattern Recognition Receptors (PRRs)?

Pattern Recognition Receptors (PRRs) are proteins on immune cells that recognize Pathogen-Associated Molecular Patterns (PAMPs) found on microbes. PRRs include:

• Toll-like receptors (TLRs) – Recognize bacterial/viral components.
• NOD-like receptors (NLRs) – Detect intracellular pathogens.
• RIG-I-like receptors (RLRs) – Identify viral RNA.

PRRs activate immune responses, including inflammation and cytokine release.

Q4: What are the Two Branches of Adaptive Immunity?

Adaptive immunity is divided into:

1. Humoral Immunity – Mediated by B cells and antibodies. Effective against extracellular pathogens.
2. Cell-Mediated Immunity – Mediated by T cells. Effective against intracellular pathogens (e.g., viruses).

Q5: What is the Germ Theory of Disease?

The Germ Theory of Disease states that microorganisms (bacteria, viruses, fungi) are responsible for causing diseases. It was proposed by Louis Pasteur and Robert Koch and led to advances in hygiene, vaccines, and antibiotics.

Q6: What are Antigens and Their Types?

An antigen is a substance that triggers an immune response. Types of antigens:

• Complete antigens – Can induce and react with an immune response (e.g., proteins, polysaccharides).
• Incomplete antigens (haptens) – Cannot induce a response alone but react with antibodies when attached to a carrier protein.

Q7: What Factors Influence Antigenicity?

Antigenicity depends on:

1. Molecular size – Larger molecules are more antigenic.
2. Chemical complexity – More complex molecules stimulate stronger responses.
3. Foreignness – The more different from self, the stronger the response.
4. Degradability – Must be broken down by antigen-presenting cells (APCs).
5. Structural stability – Unstable molecules degrade and lose antigenicity.

Q8: What is Antigenicity?

Antigenicity is the ability of an antigen to bind to immune receptors (antibodies, T-cell receptors) and trigger an immune response.

Q9: Antigens and Antigenicity: An Overview

Antigens are molecules that provoke immune responses, leading to the production of antibodies or T-cell activation.

• Antigenicity refers to an antigen’s ability to be recognized and elicit a response.
• Strong antigenicity depends on factors like size, complexity, and foreignness.

Q10: Complete vs. Incomplete Antigens

Q11: The Role of Haptens in Immunology

Haptens are small molecules that cannot trigger an immune response alone but become antigenic when attached to a carrier protein. They are significant in:

• Drug allergies (e.g., penicillin allergy).
• Vaccine development (conjugated vaccines).

Q12: Significance of Haptens and Carrier Proteins

• Haptens alone are too small to trigger immunity.
• When linked to carrier proteins, they form immunogenic complexes.
• Used in vaccine development and drug reaction studies.

Q13: Cells Involved in Antigen Processing

Cells that process and present antigens to T-cells include:

• Dendritic cells (most effective APCs).
• Macrophages (engulf pathogens and present antigens).
• B cells (present antigens to helper T-cells).

Q14: Different Classes of Antibodies

1. IgG – Most abundant, provides long-term immunity.
2. IgA – Found in secretions (tears, saliva, breast milk).
3. IgM – First antibody produced in response to infection.
4. IgE – Involved in allergic reactions.
5. IgD – Role in B cell activation.

Q15: The Role of Memory Cells in Immunity

Memory cells (B and T cells) store information about previous infections.

• On re-exposure, they trigger a faster and stronger immune response.
• Provide long-term immunity against diseases.

Q16: What is Secondary Immune Response?

• Occurs upon re-exposure to the same antigen.
• Faster and stronger due to memory cells.
• Mostly involves IgG antibodies.

Q17: Hypersensitivity Reactions Defined

Hypersensitivity reactions are exaggerated immune responses that cause tissue damage and lead to allergic reactions or autoimmune diseases.

Q18: Types of Hypersensitivity Reactions

1. Type I – Immediate (Allergic reactions, IgE mediated).
2. Type II – Cytotoxic (Antibody-mediated, e.g., hemolytic anemia).
3. Type III – Immune complex-mediated (e.g., lupus, serum sickness).
4. Type IV – Delayed-type (T-cell mediated, e.g., tuberculosis test reaction, poison ivy rash).

Q19: The Role of Cytokines in Immune Response

Cytokines are signaling proteins that:

• Activate immune cells (e.g., interleukins).
• Induce inflammation (e.g., TNF-alpha).
• Regulate immune responses (e.g., interferons).

Q20: The Role of Antigen-Presenting Cells

Antigen-presenting cells (APCs):

• Capture, process, and present antigens to T cells.
• Activate the adaptive immune system.
• Examples: Dendritic cells, macrophages, B cells.

Q21: Type 1 Hypersensitivity Reactions

Type 1 hypersensitivity (immediate allergic reaction) is caused by IgE antibodies binding to mast cells, leading to histamine release. It causes symptoms like itching, swelling, asthma, or anaphylaxis (severe allergic reaction).