Lesson 13: The Adaptive Immune System

Lesson 13: The Adaptive Immune System

General Overview

  • Adaptive immunity involves specialized lymphocytes (B cells and T cells) that coordinate to eliminate or prevent pathogenic growth and tumor cells.

  • Enhanced response due to previous exposure to infectious pathogens.

Key Characteristics of Adaptive Immunity

  • Creates immunological memory after initial response to specific pathogens.

  • Allows for quick and specific recognition of pathogens, initiating a stronger and faster secondary immune response.

  • Immunological memory can last a lifetime.

Antigen Recognition

Molecular Recognition

  • The recognition mechanism in the adaptive immune system depends on diverse receptors.

  • Each receptor recognizes a specific feature of a pathogen known as an antigen, which triggers responses from B cells and/or T cells.

Antigen Binding

  • Antigen Recognition Process: A B cell or T cell binds to an antigen through a protein called an antigen receptor.

  • B cells and T cells produce millions of different antigen receptors, yet all receptors produced by a single B cell or T cell are identical (approximately 100,000).

  • The part of the antigen that binds is known as the epitope.

B Cells and Their Activation

Activation Mechanism

  • B cells become activated in lymph nodes or spleen when their antigen receptors (BCRs) bind to a specific antigen.

  • BCRs are Y-shaped structures made of four polypeptide chains (two heavy and two light chains) linked by disulfide bridges.

Structure of BCRs

  • Each chain contains a constant region (identical among B receptors) and a variable region (located at the tips of the Y, facilitating antigen binding).

Antigen Binding Characteristics

  • BCRs can bind to intact, undigested antigens; this can occur when antigens are free in lymph or part of a pathogen.

  • Sometimes, B cells are activated by antigen presenting cells (APCs), which is crucial for T cell activation.

T Cells and Their Activation

T Cell Structure

  • Upon maturation in the thymus, T cells possess a unique antigen receptor (TCR) composed of two different polypeptide chains (α and β chains) linked via a disulfide bridge.

  • The TCR binds to epitopes from antigen fragments only when presented by MHC molecules on host cells.

Antigen Presentation Process

  • Antigen Presentation: The combination of the antigen and an MHC protein is displayed on the surface of a host cell for T cell recognition.

  • Key antigen presenting cells include macrophages and dendritic cells that phagocytize pathogens, then present their fragments with MHC.

Types of T Cells

  • Helper T Cells (Th cells): Activated by macrophages and dendritic cells, releasing cytokines that stimulate immune responses, including B cell activation and cytotoxic T cell activation.

  • Cytotoxic T Cells (Tc cells): Activated by infected cells or cytokines from Th cells. These cells kill infected or damaged cells directly through granule secretion.

Mechanisms of Action of B Cells and Antibodies

B Cell Cloning and Differentiation

  • After recognizing an antigen, the specific B cell undergoes mitosis, creating numerous clones (clonal selection theory).

  • B cells differentiate into memory B cells (long-lived, responsible for quick responses upon reinfection) and plasma cells (short-lived, active in antibody production).

Antibody Function

  • Antibodies: Secreted form of activated BCRs, capable of neutralizing or marking pathogens for destruction.

  • Antibodies do not kill pathogens directly but interfere with their function or mark them for destruction by immune cells.

  • Mechanisms:

    • Opsonization: Marking pathogens for uptake by phagocytes.

    • Binding to toxins or pathogens, often leading to the formation of clumps that can be filtered out of blood.

Immunity Types

Active Immunity

  • Acquired through natural infection or artificially through vaccinations, which stimulate a primary immune response without causing the disease.

Passive Immunity

  • Transferred from one individual to another; can be natural (e.g., from mother to infant) or artificial (e.g., through medical treatment).

Additional Resources

  • Adaptive immune system articles from reliable sources (e.g., Boundless Anatomy and Physiology, Khan Academy).