Adaptive Immunity: The Third Line of Defense – Microbiology Study Notes

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Adaptive Immunity: The Third Line of Defense

Introduction to Adaptive Immunity

Adaptive immunity represents the third line of defense in the immune system, providing acquired, specific protection against pathogens following exposure. This system is primarily mediated by B lymphocytes and T lymphocytes, which undergo a selective process to react to unique markers on foreign substances.

Acquired Specific Immunity: Develops after immunizing events such as infection or vaccination.
Immunocompetence: The ability of the body to react with countless foreign substances.
Key Cells: B and T lymphocytes are the main cellular components.

Key Characteristics of Adaptive Immunity

Specificity: Highly specific to the antigen against which the third line of defense is directed.
Diversity: Ability to respond to a wide variety of antigens.
Inducibility: Only activated in response to specific antigens.
Clonality: Rapid mobilization and proliferation of lymphocytes programmed to recall their first engagement with the invader.
Tolerance: Immune system does not attack self-antigens under normal conditions.
Memory: Enhanced response upon subsequent exposure to the same antigen.

Antigens and Immunogens

Definition and Properties

Antigens (also called immunogens) are substances that provoke an immune response. They are typically proteins or polysaccharides found on the surface of pathogens, but can also include environmental chemicals.

Antigen: Any molecule that can be recognized by the immune system and elicit a response.
Immunogen: An antigen that has been responded to by the immune system.
Examples: Viral proteins, bacterial cell wall components, toxins, and allergens.

PAMPs Versus Immunogens

PAMPs (Pathogen-Associated Molecular Patterns): Stimulate an innate immune response.
Immunogens: Highly individual, stimulate adaptive immunity.
Both are parts of foreign cells and provoke a defensive reaction from the host.

Stages of Immunologic Development and Response

Step 1: Development of Lymphocyte Diversity

Lymphocytes develop and differentiate to recognize specific antigens. This process ensures a diverse repertoire of immune cells capable of responding to a wide array of pathogens.

B cells: Mature in the bone marrow.
T cells: Mature in the thymus.

Step 2: Presentation of Antigens

Antigens must be processed and presented to lymphocytes by antigen-presenting cells (APCs) such as macrophages, dendritic cells, and B cells. These cells display antigen fragments on their surface using MHC molecules.

MHC Class I: Found on all nucleated cells; presents antigens to cytotoxic T cells.
MHC Class II: Found on certain white blood cells; presents antigens to helper T cells.

Step III: Antigenic Challenge of T Cells and B Cells

Upon encountering their specific antigen, B and T cells proliferate and differentiate into effector and memory cells.

Clonal Selection: Proliferation of genetically identical cells, some of which become memory cells.

Step IV (1): The T-Cell Response

T cells mediate cellular immunity and are activated by antigen presentation via MHC molecules.

Helper T cells (CD4+): Activate macrophages, assist B-cell processes, and help activate cytotoxic T cells.
Regulatory T cells: Control the T-cell response by secreting anti-inflammatory cytokines or preventing proliferation.
Cytotoxic T cells (CD8+): Destroy infected host cells and foreign cells.

Step IV (2): The B-Cell Response

B cells are responsible for humoral immunity, producing antibodies that neutralize pathogens.

Plasma cells: Differentiated B cells that secrete large quantities of antibodies.
Memory B cells: Provide long-term immunity by responding rapidly upon re-exposure to the antigen.

Major Histocompatibility Complex (MHC)

Role and Types

The Major Histocompatibility Complex (MHC) is a set of genes coding for cell markers that play a vital role in self-recognition and immune response.

MHC Class I: Present on all nucleated cells; interacts with CD8+ T cells.
MHC Class II: Present on antigen-presenting cells; interacts with CD4+ T cells.

Lymphocyte Receptors and Tolerance

B-Cell and T-Cell Receptors

B-Cell Receptor (Immunoglobulin): Y-shaped molecule composed of two heavy and two light chains, with variable regions for antigen binding.
T-Cell Receptor: Similar to B-cell receptor but never secreted; recognizes antigens presented by MHC molecules.

Immune Tolerance

Clonal Deletion: Removal of self-reactive lymphocytes during development to prevent autoimmunity.
Loss of tolerance: Can lead to autoimmune diseases.

Antigen Processing and Presentation

Antigen-Presenting Cells (APCs)

Macrophages, dendritic cells, and B cells process and present antigens to T cells using MHC molecules.
APCs digest pathogens and display antigen fragments for recognition by T cells.

Cell-Mediated Immunity

T-Cell Subsets and Functions

Helper T cells (TH1, TH2, TH17): Coordinate immune responses by activating other immune cells.
Regulatory T cells: Suppress immune responses to maintain homeostasis.
Cytotoxic T cells: Kill infected or abnormal cells using perforins and granzymes, leading to apoptosis.
Gamma-Delta T cells: Respond quickly to certain pathogens and produce memory cells.
Natural Killer (NK) cells: Attack cancer and virus-infected cells without antigen specificity.
Natural Killer T (NKT) cells: Share properties of both T cells and NK cells; produce cytokines and cytotoxic molecules.

Immunoglobulins (Antibodies)

Structure and Function

Immunoglobulin (Ig) Molecule: Composed of polypeptide chains connected by disulfide bonds, with antigen-binding fragments (FAbs) and a crystallizable fragment (Fc).
Variable region: Binds to the epitope of the antigen; highly diverse.

Classes of Immunoglobulins

Class	Function	Location
IgG	Long-term immunity, most abundant	Blood, extracellular fluid
IgA	Secretory antibody	Tears, saliva, mucous membranes
IgM	First antibody produced	Blood
IgD	Receptor on B cells	B cell surface
IgE	Allergic responses, parasitic infections	Blood, tissues

Primary and Secondary Immune Responses

Primary response: Initial exposure to antigen; slower, lower concentration of antibodies.
Secondary response: Subsequent exposure; rapid, high concentration of antibodies due to memory cells.

Vaccination and Immunity

Types of Immunity

Active Immunity: Acquired through infection or vaccination; long-lasting.
Passive Immunity: Acquired through transfer of antibodies (e.g., breastfeeding, intravenous immunoglobulin); short-term.

Types of Vaccines

Type	Description	Example
Killed/Inactivated	Contains killed pathogens	Polio (Salk), influenza
Live, Attenuated	Contains weakened pathogens	MMR, varicella
Subunit/Component	Contains only antigenic parts	Hepatitis B, HPV
mRNA	Encodes viral protein, delivered in lipid vesicle	COVID-19 (Pfizer, Moderna)
Viral Vector	Uses harmless virus to deliver antigen gene	COVID-19 (AstraZeneca, J&J)

Principles of Vaccine Preparation

Should have low adverse effects and high effectiveness.
Stimulate both antibody and cell-mediated responses.
Produce long-term immunity with minimal doses.
Be inexpensive and easy to administer.

Booster Shots

Additional doses to maintain long-term immunity.
Not always required if initial vaccine provides lifelong protection.

Vaccine Side Effects and Misinformation

Common side effects: Local soreness, short-term flu-like symptoms, rare risk of anaphylaxis.
Misinformation: Myths about vaccines causing disease or autism are unfounded and contribute to vaccine hesitancy.

Recommended Immunization Schedules

Most intense period for children is between birth and 15 months.
Adults may require boosters or vaccines for specific infections.

Additional info:

CRISPR and genetic engineering are being explored for future vaccine development.
Adjuvants are added to some vaccines to enhance immune response.