Adaptive Immunity: Structure, Function, and Mechanisms

Overview of Adaptive Immunity

Introduction to Adaptive Immunity

Adaptive immunity is the body's highly specific defense mechanism against distinct pathogens and their products. It is characterized by its ability to recognize, remember, and mount stronger attacks on previously encountered invaders.

• Specificity: Targets unique antigens.

• Inducibility: Activated in response to specific pathogens.

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

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

• Memory: Remembers previous encounters for faster future responses.

Adaptive immunity involves two main types of lymphocytes: B lymphocytes (B cells) and T lymphocytes (T cells). These cells mediate two types of immune responses: humoral (antibody-mediated) and cell-mediated immunity.

Types of Adaptive Immune Responses

• Humoral immunity: Mediated by B cells and the antibodies they produce. Effective against extracellular pathogens.

• Cell-mediated immunity: Mediated by T cells, particularly effective against intracellular pathogens such as viruses and some bacteria.

Elements of Adaptive Immunity

The Lymphatic System

The lymphatic system screens body tissues for foreign antigens and is composed of lymphatic vessels, cells, tissues, and organs. It is a one-way system that returns lymph (a fluid similar to blood plasma) to the circulatory system.

• Primary lymphoid organs: Red bone marrow (site of B cell maturation) and thymus (site of T cell maturation).

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

Antigens

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

• Can be components of bacteria, viruses, fungi, protozoa, or even non-microbial substances like food and dust.

• Each antigen may have multiple epitopes, each recognized by a different immune receptor.

B Lymphocytes (B Cells) and Antibodies

B cells arise and mature in the red bone marrow and are primarily found in the spleen, lymph nodes, and MALT. Their main function is the secretion of antibodies.

• B cell receptor (BCR): Each B cell has a unique BCR that binds a specific epitope. The diversity of BCRs allows recognition of millions of different antigens.

• Antibody structure: Antibodies (immunoglobulins) are secreted by plasma cells (activated B cells) and have the same specificity as the BCR of the parent B cell.

• Antibody functions:

  • Activation of complement and inflammation

  • Neutralization of toxins and pathogens

  • Opsonization (enhancing phagocytosis)

  • Killing by oxidation

  • Agglutination (clumping of antigens)

  • Antibody-dependent cellular cytotoxicity (ADCC)

• Classes of antibodies:

  • IgM: First antibody produced during an immune response.

  • IgG: Most common and long-lasting antibody in serum.

  • IgA: Associated with body secretions (e.g., mucosal immunity).

  • IgE: Involved in responses to parasitic infections and allergies.

  • IgD: Function not fully understood.

T Lymphocytes (T Cells)

T cells are produced in the red bone marrow and mature in the thymus. They circulate in the lymph and blood and migrate to secondary lymphoid organs. T cells have T cell receptors (TCRs) that recognize antigens only when presented with major histocompatibility complex (MHC) proteins.

• Types of T lymphocytes:

  • Cytotoxic T lymphocytes (Tc): Directly kill infected or abnormal cells.

  • Helper T lymphocytes (Th): Regulate activities of B cells and Tc cells via cytokine secretion.

  • Regulatory T lymphocytes (Tr): Suppress immune responses to prevent autoimmunity.

Clonal Deletion

Clonal deletion is a process that eliminates self-reactive lymphocytes to prevent autoimmune responses. Lymphocytes that recognize self-antigens undergo apoptosis (programmed cell death).

Immune Response Cytokines

Cytokines are soluble regulatory proteins that act as intercellular signals in the immune system. They include:

• Interleukins (ILs): Signal among leukocytes.

• Interferons (IFNs): Antiviral proteins that may act as cytokines.

• Growth factors: Stimulate stem cell division.

• Tumor necrosis factor (TNF): Kills tumor cells and regulates immune responses.

• Chemokines: Induce chemotaxis of leukocytes.

Preparation for an Adaptive Immune Response

Major Histocompatibility Complex (MHC)

MHC proteins are glycoproteins found on the membranes of most vertebrate cells. They present antigenic peptides to T cells and are crucial for immune recognition and tissue compatibility.

• MHC class I: Present on all nucleated cells; present endogenous antigens.

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

Antigen Processing and Presentation

Antigens must be processed and presented by MHC molecules for recognition by T cells. The process differs for endogenous (intracellular) and exogenous (extracellular) antigens.

• Endogenous antigens: Processed within infected cells and presented on MHC I molecules.

• Exogenous antigens: Processed by APCs and presented on MHC II molecules.

Cell-Mediated Immune Responses

Activation of T Cell Clones

Cell-mediated immunity is primarily directed against intracellular pathogens and abnormal cells. The activation of cytotoxic T cells involves several steps:

1. Antigen presentation

2. Helper T cell differentiation

3. Clonal expansion

4. Self-stimulation

Memory T Cells

Some activated T cells become memory T cells, which persist in lymphoid tissues and respond rapidly upon re-exposure to their specific antigen.

T Cell Regulation

Regulation is essential to prevent T cell responses against self-antigens. T cells require additional signals from antigen-presenting cells to become fully activated.

Humoral Immune Responses

Activation of B Cells

Humoral immune responses are mounted against exogenous pathogens and involve the activation and proliferation of B cells. B cells can be activated independently (T-independent antigens) or with T cell help (T-dependent antigens).

• Plasma cells: Short-lived cells that secrete antibodies specific to the antigen.

• Memory B cells: Long-lived cells that do not secrete antibodies but can rapidly respond to future exposures to the same antigen.

Types of Acquired Immunity

Classification of Acquired Immunity

Acquired immunity can be classified based on how it is obtained:

Type	Active	Passive
Naturally acquired	Response to antigens encountered in daily life	Antibodies transferred from mother to child
Artificially acquired	Response to antigens introduced via vaccination	Antibodies received from immune serum

Active immunity involves the production of antibodies by the individual's own immune system, while passive immunity involves receiving antibodies from another individual.