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Presentation of the Antigen

  • Vaccination

  • Immune Response:

    • Virus antigens processed and presented to T cells by antigen-presenting cells (APCs).

    • MHC-I and MHC-II molecules play critical roles in immune cell interactions.

    • T cells include:

      • Cytotoxic T cells (CD8, Tc): referred to as "The Hitman" - responsible for killing infected cells.

      • Helper T cells (CD4, TH): referred to as "The General" - crucial for orchestrating the immune response through cytokines such as Interleukin-1 and Interleukin-2.

  • Specific Immunity:

    • Involves an adaptive immune response with memory cells for faster reactions upon re-exposure to antigens.

    • Illustrations depict T cell dynamics and response.

Overview of Host Defenses

  • Three Lines of Defense:

    1. First Line: Non-specific, innate defenses that block foreign entities.

    2. Second Line: Innate responses that include immune cells and inflammation.

    3. Third Line: Specific, adaptive immunity acquired through infection or vaccination, involving B and T lymphocytes.

  • Types of White Blood Cells:

    • Granulocytes (large cytoplasmic granules, lobed nucleus), e.g., neutrophils.

    • Agranulocytes (small granules, rounded nucleus), e.g., lymphocytes and monocytes.

Specific Immunity and Acquired Responses

  • Characteristics:

    • Immunocompetence: The ability of the body to respond to foreign substances.

    • Lymphocyte Development: Involves the generation of B and T lymphocytes which are specialized to react to distinct antigens.

    • Specificity and Memory: Key features of the adaptive immune system.

  • Five Main Stages of Immunologic Development:

    1. Lymphocyte development and differentiation.

    2. Antigen presentation.

    3. Challenge of B and T lymphocytes by antigens.

    4. Responses by mature lymphocytes.

    5. Cell-mediated and humoral immunity provided by T cells and B cells, respectively.

Lymphocyte Development

  • All lymphocytes originate from hematopoietic stem cells in bone marrow.

  • B Cells: Mature in bone marrow, migrate to lymphoid organs.

  • T Cells: Mature in thymus, also migrate to lymphoid organs, engaging closely with B cells during immune responses.

  • Antigen presentations stimulate both B and T cells leading to clonal responses.

Antigen Processing and Presentation

  • Antigen-presenting Cells (APCs):

    • Include macrophages, dendritic cells, and B cells.

    • Process and present antigens to T cells, facilitating immune activation.

  • MHC Molecules: Major Histocompatibility Complex (HLA) critical for self/non-self identification.

    • Class I: Display characteristics of self to cytotoxic T cells.

    • Class II: Immune regulatory receptors found on professional APCs displaying antigens to helper T cells.

B Cell Receptors and Specificity

  • Immunoglobulins:

    • Y-shaped molecules that serve as B cell receptors and antibodies once secreted.

    • Include variable regions (for antigen specificity) and constant regions (for effector functions).

  • Antibody Structure:

    • Comprises 4 polypeptide chains linked by disulfide bonds, forming two antigen-binding arms.

    • B cell receptors bind specific antigens, marking them for destruction via various mechanisms (e.g., agglutination).

T Cell Response to Antigens

  • Activation and Progeny:

    • Upon binding to the antigen-MHC complex, T cells initiate rapid mitotic division forming:

      • Memory T cells - faster response upon re-exposure.

      • Helper T cells (TH) - coordinate immune responses.

      • Cytotoxic T cells (TC) - kill infected or aberrant cells.

    • Cytokines, such as IL-2, stimulate the proliferation and activity of other immune cells.

Regulatory and Cytotoxic T Cells

  • Regulatory T Cells (TR):

    • Maintain immune balance, preventing autoimmunity and inhibiting undesirable responses.

  • Cytotoxic T Cells (TC):

    • Kill cells infected by pathogens, particularly those expressing foreign antigens in conjunction with MHC-I.

  • Natural Killer (NK) Cells:

    • Non-specific, rapid response to stressed or infected cells without the need for prior sensitization.

Principles of Vaccine Preparation

  • Effective vaccines must:

    • Have low adverse side effects.

    • Protect against natural pathogens.

    • Produce both antibody and cell-mediated responses.

    • Offer long-term immunity and require minimal boosters.

    • Be cost-effective and easily administered.

Types and Mechanisms of Vaccines

  • Types of Vaccines:

    • Live attenuated (weakened pathogens) vs. inactivated (killed pathogens).

    • Subunit and conjugated vaccines (focus on specific parts of pathogens).

  • Inactivated vs. Attenuated:

    • Inactivated vaccines are safer but may offer weaker responses compared to live vaccines.

New Vaccine Development Challenges

  • Addressing persistent viral challenges.

  • Utilizing genetically engineered strategies to enhance vaccine effectiveness.

Immunity Definitions

  • Acquired Immunity:

    • Natural Immunity: Achieved through natural exposures.

    • Active Immunity: Develops through infection or vaccination.

    • Passive Immunity: Transfer of pre-formed antibodies (short-lived).

    • Artificial Immunity: Induced through medical intervention, often via vaccination.