4.4 Innate vs Adaptive Immunity: Adaptive Immunity

Introduction

  • The video is a continuation of a two-part series presented by Dr. Vanessa, focusing on immunity in biology.

  • Part one covered innate immunity; part two will cover adaptive immunity and include a comparison of both systems.

Innate Immunity Recap

  • Innate immunity is characterized as:

    • Non-specific: It does not target specific pathogens.

    • Rapid response: Acts quickly upon pathogen exposure.

  • Covers mechanisms that act immediately to prevent infection.

Overview of Adaptive Immunity

  • Adaptive immunity develops if innate immunity is insufficient to eliminate pathogens.

  • Also known as:

    • Acquired immunity

    • Programmed immunity

  • Key characteristics:

    • Specific: Aimed at particular non-self antigens.

    • Involves memory cells: Enhances future responses to previously encountered antigens.

Definition of Antigen

  • An antigen is defined as a substance recognized as foreign by the body, provoking an immune response.

Self vs. Non-Self Recognition

  • Adaptive immunity distinguishes between self and non-self proteins through:

    • Basic training: Cells learn to recognize self-antigens and avoid attacking them (self-tolerance).

    • If self-tolerance is not mastered, the cells are destroyed to prevent autoimmune diseases.

Loss of Self-Tolerance

  • When adaptive immune cells begin to recognize self-proteins, it can lead to autoimmune diseases.

Components of the Adaptive Immune System

  • Lymphocytes (a type of white blood cell) play a central role. The two main types:

    • B lymphocytes (B cells)*: Mature in bone marrow.

    • T lymphocytes (T cells): Mature in the thymus.

Origin of Lymphocytes

  • Lymphocytes originate from:

    • Hematopoietic stem cells in red bone marrow.

  • Differentiation pathways:

    • Hematopoietic stem cell → Common myeloid progenitor (produces red blood cells, progenitor of innate immune cells)

    • Hematopoietic stem cell → Common lymphoid progenitor (produces lymphocytes: T cells, B cells, and natural killer cells).

Types of Immunity Derived from Lymphocytes

  • Two types of adaptive immunity stemming from the activity of B cells and T cells:

    1. Cell-mediated immunity:

    • Involves T cells, specifically cytotoxic T cells, which attack invading pathogens directly.

    1. Antibody-mediated immunity:

    • Involves B cells recognizing antigens, activating into plasma cells that secrete antibodies.

Role of Helper T Cells

  • Helper T cells assist in both cell-mediated and antibody-mediated immunity by:

    • Helping activate B cells and cytotoxic T cells.

Specificity of Immune Response

  • Adaptive immune responses are highly specific, contrasting with non-specific innate immunity.

  • During the immune response, memory cells are generated, allowing for quicker responses to previously encountered pathogens.

Major Histocompatibility Complex (MHC)

  • MHC molecules are essential for recognizing self and non-self antigens.

  • Types of MHC:

    1. MHC Class I (MHC I):

    • Found on all body cells except red blood cells.

    • Displays endogenous antigens (produced within the cell).

    • Indicates cell health to immune cells (healthy cells show normal proteins; infected cells show non-self proteins leading to immune attack).

    1. MHC Class II (MHC II):

    • Found solely on antigen-presenting cells (APCs) capable of phagocytosis.

    • Displays exogenous antigens (taken from outside the cell).

Activation of Cytotoxic T Cells (Cell-Mediated Immunity)

  • Cytotoxic T cells are activated when they:

    • Recognize MHC I displaying non-self proteins on infected body cells.

    • Bind to the MHC-1 antigen complex triggering activation and clonal expansion, which produces activated and memory T cells.

  • Activated T cells release perforin and granzymes to induce apoptosis in infected cells.

Activation of B Cells (Antibody-Mediated Immunity)

  • B cells can be activated in two main ways:

    1. By free-floating antigens:

    • B cell receptors initiate an immune response upon binding with antigens, leading to internalization and clonal expansion (producing memory B cells and plasma cells).

    1. By T helper cells:

    • Activated helper T cells can bind to B cells to facilitate their activation and expansion.

  • Plasma cells secrete antibodies matching the specific antigen.

Antibody Functions

  • Antibodies can:

    • Bind to and neutralize antigens.

    • Opsonize antigens for better recognition and destruction by other immune cells.

Comparisons Between Innate and Adaptive Immunity

  • Innate Immunity:

    • Non-specific, rapid response.

    • Cells involved: mast cells, neutrophils, monocytes, dendritic cells, macrophages, and natural killer cells.

  • Adaptive Immunity:

    • Specific, initially slower but faster on subsequent exposures.

    • Cells involved: B cells and T cells (cytotoxic and helper T cells).

Conclusion

  • Video expresses gratitude for audience engagement and encourages further comments and questions.

  • Viewers are invited to suggest future topics for discussion.