Lecture on Immune Response and Memory Cells

Immune Response Overview

B Cells and Antibodies

  • B Cells Proliferation: B cells proliferate upon activation by an antigen, resulting in an increase in their numbers.

  • Maturation: Some B cells mature into plasma cells that secrete antibodies, while others become memory B cells.

    • Plasma Cells: Characterized by the production of antibodies that are specific to the antigen (represented as red dots in the discussion).

    • Memory B Cells: Long-lived cells that provide faster immune responses upon subsequent exposures to the same antigen.

    • Duration of Memory Cells: Memory B cells can last from months to years; specific lifespan varies per cell and type of pathogen.

Memory Cell Response

  • Upon re-exposure to the same infection, memory B cells trigger a quicker and more robust antibody response, compared to the initial exposure.

  • Example: After initial exposure to an antigen (e.g., red dots), it may take about 5-6 days for the immune system to generate a sufficient antibody response. In case of reinfection, antibodies can be produced almost immediately, and at a significantly higher concentration.

Primary and Secondary Immune Responses

  • Primary Immune Response: Longer response time with multiple steps the first time an antigen is encountered.

  • Secondary Immune Response: Faster and amplified due to the presence of memory B cells. This response only works if the same antigen is encountered (no mutations).

Importance of Antigens and Epitopes

  • Antigen Structure: Antigens consist of multiple epitopes. For instance, various forms of the coronavirus have different epitopes. The immune system targets these epitopes.

  • Antigen Mutation: If a virus mutates and the new strain has significantly different epitopes, pre-existing memory cells may not respond effectively, although partial immunity could remain if some epitopes are conserved.

T Cells and Their Activation

  • T Cell Receptors (TCRs): T cells uniquely recognize specific antigens presented to them by Antigen Presenting Cells (APCs) through MHC (Major Histocompatibility Complex) proteins.

  • Types of T Cells:

    • Helper T Cells (CD4+ T Cells): Activate B cells and other T cells through cytokine signaling. They are essential for effective immune responses.

    • Cytotoxic T Cells (CD8+ T Cells): Directly destroy infected or cancerous cells.

    • Regulatory T Cells (Tregs): Modulate the immune response to prevent excessive reactions or autoimmunity.

Antigen Presentation

  • MHC Class I: Presents endogenous antigens (from inside infected cells). Found on nearly all nucleated cells.

  • MHC Class II: Presents exogenous antigens (from outside sources). Found predominantly on APCs like dendritic cells and macrophages.

Functions of Antibodies

  • Antibodies, also known as immunoglobulins, facilitate immune responses through various mechanisms:

    • Precipitation: Isolating soluble antigens by causing them to fall out of solution, making it easier for the immune system to deal with them.

    • Lysis: Facilitating the destruction of pathogens through complement activation or marking them for destruction (opsonization).

    • Antibodies bind to antigens and trigger complement proteins that lyse cells.

    • Agglutination: Clumping of pathogens or foreign particles to enhance phagocytosis.

    • Neutralization: Blocking the biological activity of toxins or pathogens by binding to them.

Immunoglobulin Classes

  • Five Classes of Immunoglobulins: IgM, IgG, IgA, IgE, and IgD, each with distinct functions and structures (e.g., monomers or pentamers). All share a basic Y-shape structure with a constant region and variable regions for specificity against different antigens.

Vaccination and Immunity

  • Types of Vaccines: Vaccines may be live attenuated, inactivated, or composed of specific proteins (e.g., spike proteins).

  • Mechanism of Vaccines: Vaccination leads to primary immune responses, aimed at generating memory cells to provide long-lasting protection.

  • mRNA Vaccines: Recent developments using lipid nanoparticles to deliver mRNA encoding for antigens, eliciting strong immune responses without the risk of disease caused by the virus due to the inactivated or absent viral components.

Immune System Deficiencies

  • Primary: Often genetic, leading to congenital defects in immune responsiveness.

  • Secondary: Resulting from environmental factors, illness, or medical treatments (e.g., chemotherapy affecting immune cell production).

CAR T Cell Therapy

  • Chimeric Antigen Receptor T Cell (CAR T): A revolutionary therapy where T cells are genetically modified to better recognize and target cancer cells. This includes removing their DNA, inserting instructions for specific receptors, and reintroducing them to patients where they proliferate to attack cancer cells.

Clinical Applications

  • Antiserum: A serum containing antibodies specific to certain antigens for neutralizing toxins or pathogens in acutely ill patients. However, it does not provoke long-term immune memory.

  • Genetic Engineering: Potential for correcting congenital defects or developing targeted therapies for various diseases, as exemplified through CAR T cell therapies for cancer, and research into using stem cells for genetic corrections in immune deficiencies.