Adaptive Immunity Overview

Adaptive Immunity Lecture Notes

• Adaptive immunity is not innate; it develops throughout life.
• It's slower than the innate system but highly specific.
• It has a memory function, adapting to repeated exposures.

Learning Objectives

• Understanding the features of adaptive immunity.
• The 3 Rs of adaptive immunity (Recognize, Respond, Remember).
• Clonal selection.
• Cell-mediated immunity.
• 3 types of T cells.
• Antigen presentation.
• Antibody-mediated (humoral) immunity.
• B cells.
• Antibody structure and function.
• Primary and secondary antibody responses.

Cells Involved

• Lymphocytes are central to adaptive immunity.
  • B lymphocytes produce antibodies.
  • T lymphocytes are involved in cell-mediated immunity.
• T cells mature in the thymus.
• B cells mature in the bone marrow.

Principles of Adaptive Immunity

• Each lymphocyte has a specific receptor for a particular antigen.
• Innate immune cells help with recognition.
• This recognition is followed by clonal selection.
• Adaptive responses include cytokine secretion, antibody production, and direct cell killing.
• Memory cells are crucial for quicker responses in subsequent encounters with the same antigen.

Clonal Selection

• (Burnett, 1957) Lymphocytes have receptors to bind a single antigen.
• Lymphocyte repertoire is highly diverse with varied specificity.
• When an activated lymphocyte encounters its antigen, it becomes activated and multiplies rapidly to create a clone of effector and memory cells.
• Effector cells directly combat the antigen.
• Memory cells provide a rapid response to subsequent exposures.

Clonal Selection (Diagram Explanation)

• Primary response: Recognition and activation of naïve B cells, followed by the formation of clones and antibody production by plasma cells.
• Secondary response: Memory B cells rapidly produce large amounts of antibody upon re-exposure; the antibody response is much faster, stronger, and more focused.

Cell-mediated Immunity

• T cells require antigen-presenting cells (APCs) to process and present antigens.
• APCs include phagocytes, dendritic cells, and macrophages.
• APCs display digested antigen fragments on their MHC (Major Histocompatibility Complex).
• This presentation activates T cells.
• After activation, helper T cells enhance the immune response, cytotoxic T cells directly kill infected cells, and regulatory T cells suppress unnecessary responses.

Antigen Presentation (Detailed)

• Antigens invade the body.
• Phagocytosis by antigen-presenting cells (APCs).
• Digestion of antigen into smaller particles.
• Transport of these particles to the cell surface.
• Display on surface via MHC molecules.

T Cell Types

• Regulatory T cells (Tregs): Suppress immune responses.
• Helper T cells: Activate B cells and other immune cells.
• Cytotoxic T cells: Kill infected host or foreign cells.

B Cells & Antibodies (Humoral Immunity)

• B cell receptors (BCRs) are antibodies that directly bind antigens.
• BCRs recognize specific epitopes (antigenic determinants) on antigens.
• After clonal selection and expansion, B cells produce antibodies with the same specificity as their BCRs.

Antibodies

• Antibodies (Immunoglobulins): Y-shaped proteins with two identical heavy chains and light chains.
• Two antigen-binding sites (Fab).
• Stem (Fc) region binds to Fc receptors.
• Antibodies mediate various immune responses, including neutralizing, opsonization, and complement activation.

Antibody Structure Details

• 4 polypeptide chains with consistent and variable regions.
• Antigen-binding sites are variable.
• Stem region is constant across antibody types.

Antibody Isotypes

• Five main types (IgG, IgM, IgA, IgD, IgE) with distinct roles in immunity. IgG is a major one, present in high concentrations. IgM is used to recognize pathogens. IgA is present in secretions. IgD is found on B-cell surfaces. IgE is involved in allergies.

Functions of Antibodies

• Neutralization: Blocking pathogen binding to cells.
• Opsonization: Enhancing phagocytosis by tagging pathogens.
• Antibody-dependent cellular cytotoxicity (ADCC): NK cells killing antibody-coated cells.
• Complement activation: Cascade of proteins that enhances pathogen destruction.
• Inflammation: Initiating inflammation response.

Primary vs. Secondary Antibody Response

• Primary response: 5-10 days, low titre, mainly IgM.
• Secondary response: 1-3 days, high titre, mainly IgG, more antibody specificity, affinity maturation.

Immunological Memory

• A faster, larger, and more effective response upon subsequent antigen encounters due to memory cells.

Summary of Main Points

• Adaptive immunity is specific, adaptable, and has immunological memory.
• Clonal selection creates a specific immune response.
• Antigen presentation is essential for T-cell activation.
• B cells produce antibodies.
• Different antibody isotypes have unique functions.
• Secondary responses are superior to primary responses due to the presence of memory cells.