Adaptive Immunity

Introduction to Adaptive Immunity

Adaptive immunity is the body's ability to recognize and defend itself against specific invaders and their products. It is a highly specialized and specific response that develops after exposure to antigens.

• Specificity: Targets specific pathogens or foreign substances.

• Inducibility: Activated in response to specific antigens.

• Clonality: Generates clones of lymphocytes specific to the antigen.

• Unresponsiveness to self: Normally does not attack the body's own cells.

• Memory: Provides long-lasting protection through memory cells.

Comparison of Innate and Adaptive Immunity

The immune system is divided into innate and adaptive branches, each with distinct characteristics:

Overview of Adaptive Immunity

Lymphocytes and Immune Responses

Adaptive immunity involves the activity of lymphocytes, which are white blood cells specialized for immune defense. There are two main types:

• B lymphocytes (B cells): Mature in the bone marrow; responsible for antibody production.

• T lymphocytes (T cells): Mature in the thymus; responsible for cell-mediated immunity.

There are two types of adaptive immune responses:

• Cell-mediated immune responses: Involve T cells attacking infected or abnormal cells.

• Antibody (humoral) immune responses: Involve B cells producing antibodies to neutralize pathogens.

Elements of Adaptive Immunity

Tissues and Organs of the Lymphatic System

• Lymphatic vessels: One-way system that conducts lymph from tissues and returns it to the circulatory system.

• Lymph: Fluid similar to blood plasma, arising from fluid leaked from blood vessels into surrounding tissues.

• Lymphoid organs:

  • Primary lymphoid organs: Red bone marrow and thymus (sites of lymphocyte maturation).

  • Secondary lymphoid organs: Lymph nodes, spleen, tonsils, and mucosa-associated lymphoid tissue (MALT).

Antigens

Antigens are molecules recognized as foreign and capable of triggering an immune response. They are identified by specific regions called epitopes or antigenic determinants.

• Large foreign macromolecules (proteins, polysaccharides) make the best antigens.

• Sources include bacterial components, viral proteins, fungi, protozoa, food, and dust.

Types of Antigens

• Exogenous antigens: Toxins and components of microbial cell walls, membranes, flagella, and pili; present outside cells.

• Endogenous antigens: Produced by microbes that reproduce inside a body's cells.

• Autoantigens: Derived from normal cellular processes; usually ignored by the immune system.

Major Histocompatibility Complex (MHC)

MHC molecules are glycoproteins found on cell membranes that present antigenic epitopes to immune cells. They are crucial for tissue compatibility and immune recognition.

• MHC class I: Present on all nucleated cells except red blood cells.

• MHC class II: Present on antigen-presenting cells (APCs) such as macrophages, B cells, and dendritic cells.

Antigen Processing and Presentation

• Antigens are processed for MHC proteins to display epitopes on the cell surface.

• Different processes apply for endogenous (intracellular) and exogenous (extracellular) antigens.

T Lymphocytes (T Cells)

Development and Function

• Produced in the red bone marrow, mature in the thymus.

• Circulate in lymph and blood, migrate to lymph nodes, spleen, and Peyer's patches.

• Have T cell receptors (TCRs) that recognize epitopes only when presented by MHC molecules.

Types of T Lymphocytes

• Cytotoxic T lymphocyte (Tc): Directly kills infected or abnormal cells.

• Helper T lymphocyte (Th): Regulates B cells and cytotoxic T cells; includes type 1 and type 2 helper T cells.

• Regulatory T lymphocyte (Treg): Downregulates adaptive immune responses to prevent autoimmunity.

Table: Characteristics of T Lymphocytes

Clonal Deletion of T Cells

• Prevents immune responses against self-antigens (autoimmunity).

• Self-reactive T cells undergo apoptosis or become regulatory T cells.

• Only T cells that recognize foreign antigens survive to form the protective repertoire.

B Lymphocytes (B Cells) and Antibodies

Development and Function

• Found mainly in spleen, lymph nodes, and MALT; a small percentage circulate in blood.

• Major function is secretion of antibodies (immunoglobulins).

B Cell Receptor (BCR) Specificity and Diversity

• Each B cell has multiple copies of a unique BCR, recognizing only one epitope.

• Genetic recombination (via RAG enzyme) generates diversity, allowing recognition of millions of epitopes.

Antibody Structure and Function

• Antibodies are secreted by activated B cells (plasma cells).

• Antigen-binding sites are complementary to specific epitopes.

• Functions include:

  • Activation of complement and inflammation

  • Neutralization of toxins and pathogens

  • Opsonization (enhancing phagocytosis)

  • Agglutination (clumping of antigens)

  • Antibody-dependent cellular cytotoxicity (ADCC)

Classes of Antibodies

• IgM: First antibody produced in response to infection.

• IgG: Most common and longest-lasting antibody; provides the majority of antibody-based immunity.

• IgA: Associated with body secretions (e.g., saliva, tears, mucous).

• IgE: Involved in response to parasitic infections and allergic reactions.

• IgD: Function not well understood.

Summary

Adaptive immunity is a complex, highly specific defense system involving B and T lymphocytes, antigen recognition, and memory formation. It is essential for long-term protection against pathogens and is the basis for immunological memory and vaccination.