Adaptive Immunity: The 3rd Line of Defense

Overview of Adaptive Immunity

Adaptive immunity is a specific immune response that is acquired only after an immunizing event, such as an infection or vaccination. It is characterized by specificity and memory, allowing the immune system to respond more efficiently upon subsequent exposures to the same pathogen.

• Specificity: The immune system targets specific antigens.

• Memory: The immune system responds more rapidly and effectively to previously encountered antigens.

• Primary vs. Secondary Response: The first exposure to an antigen elicits a primary response, while subsequent exposures elicit a faster and stronger secondary response.

• Example: Vaccination primes the immune system to respond more effectively to future infections by the same pathogen.

Distinguishing Innate and Adaptive Immunity

• Innate Immunity: Non-specific, immediate response; no memory.

• Adaptive Immunity: Specific, delayed response; develops memory.

• Key Cells: B cells and T cells are central to adaptive immunity.

Antigens and Immunogenicity

Properties of Antigens

Antigens are molecules that can be recognized by the immune system and elicit an immune response. Not all antigens are equally effective at stimulating immunity; their ability to do so is called immunogenicity.

• Degree of Foreignness: More foreign molecules are more immunogenic.

• Molecular Size: Larger molecules are generally more immunogenic.

• Chemical Complexity: More complex molecules are better antigens.

• Degradability: Molecules that can be processed by antigen-presenting cells are more immunogenic.

Antigen Recognition by B and T Cells

• B Cell Receptors (BCR): Recognize native (unprocessed) antigens directly.

• T Cell Receptors (TCR): Recognize processed antigen fragments presented by MHC molecules on antigen-presenting cells.

• Epitope: The specific part of an antigen recognized by an antibody or TCR.

Major Histocompatibility Complex (MHC)

Classes and Functions of MHC Molecules

MHC molecules are cell surface proteins essential for antigen presentation to T cells.

• Role in Immunity: MHC molecules are critical for T cell activation and immune response regulation.

Lymphocyte Development and Activation

T Cell Development (in the Thymus)

T cells originate from hematopoietic stem cells in the bone marrow and mature in the thymus. During maturation, T cells undergo selection processes to ensure self-tolerance and functional competence.

• Positive Selection: T cells that can recognize self-MHC molecules survive.

• Negative Selection: T cells that bind too strongly to self-antigens are eliminated (prevents autoimmunity).

B Cell Development (in the Bone Marrow)

• B cells mature in the bone marrow and undergo selection to eliminate self-reactive cells.

• Mature B cells express unique B cell receptors (BCRs) specific for particular antigens.

Lymphocyte Activation

• T Cell Activation: Requires antigen presentation by APCs and co-stimulatory signals.

• B Cell Activation: Can be T cell-dependent (requires T cell help) or T cell-independent (direct activation by certain antigens).

Effector Functions of Adaptive Immunity

Antibody-Mediated (Humoral) Immunity

B cells differentiate into plasma cells that secrete antibodies. Antibodies neutralize pathogens, opsonize for phagocytosis, and activate complement.

• Antibody Structure: Y-shaped molecules with variable regions for antigen binding.

• Antibody Classes (Isotypes): IgG, IgM, IgA, IgE, IgD – each with distinct functions.

• Example: IgG is the most abundant antibody in serum and provides long-term immunity.

Cell-Mediated Immunity

• Helper T Cells (CD4+): Activate B cells, cytotoxic T cells, and macrophages.

• Cytotoxic T Cells (CD8+): Kill infected or abnormal cells presenting antigen via MHC I.

• Regulatory T Cells: Suppress immune responses to maintain tolerance.

Immunological Memory

After initial exposure to an antigen, memory B and T cells are generated. These cells persist long-term and enable a faster, stronger response upon re-exposure to the same antigen.

• Primary Response: Slower, lower magnitude.

• Secondary Response: Faster, higher magnitude due to memory cells.

Types of Immunity

Active vs. Passive Immunity

• Active Immunity: Host produces its own antibodies and memory cells after exposure to antigen (infection or vaccination).

• Passive Immunity: Host receives pre-formed antibodies from another source (maternal antibodies, antiserum).

• Natural vs. Artificial: Immunity can be acquired naturally (infection, maternal transfer) or artificially (vaccination, antibody therapy).

Vaccination

Vaccines stimulate adaptive immunity to provide protection against specific pathogens. Types include live attenuated, inactivated, subunit, toxoid, and conjugate vaccines.

• Herd Immunity: When a large portion of the population is immune, the spread of disease is reduced.

Summary Table: Types of Acquired Immunity

Key Terms and Concepts

• Antigen: Substance that elicits an immune response.

• Epitope: Specific region of an antigen recognized by immune receptors.

• Antibody: Protein produced by B cells that binds to specific antigens.

• MHC: Molecules that present antigen fragments to T cells.

• Clonal Selection: Process by which lymphocytes with specific receptors proliferate upon antigen exposure.

• Immunological Memory: Ability of the immune system to respond more rapidly and effectively to previously encountered antigens.