Adaptive Immune System: Structure, Function, and Mechanisms

Adaptive Immune System

Overview and Purpose

The adaptive immune system is a highly specialized defense mechanism that enables the body to recognize and eliminate specific pathogens and their products. It builds upon the innate immune response and is characterized by its ability to target distinct invaders with precision.

• Specificity: Targets unique molecular structures (antigens) on pathogens.

• Inducibility: Activated only in response to specific antigens.

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

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

• Memory: Remembers previous encounters for faster future responses.

Key Characteristics of Adaptive Immunity

Adaptive immunity is distinguished by four main features:

• Specificity: Recognizes and responds to specific epitopes on antigens.

• Tolerance to self: Prevents immune responses against self-antigens, failure of which leads to autoimmune diseases.

• Minimal self-damage: Immune responses are regulated to avoid excessive tissue damage.

• Immunological memory: Enables rapid and robust responses upon re-exposure to the same antigen.

Types of Leukocytes in Adaptive Immunity

Leukocytes, or white blood cells, are central to immune function. The two primary types involved in adaptive immunity are:

• B cells: Mature in bone marrow, produce antibodies (immunoglobulins) that bind to antigens.

• T cells: Mature in thymus, include helper T cells (CD4+) that direct immune responses via cytokines, and cytotoxic T cells (CD8+) that kill infected or abnormal cells.

Dendritic Cells: Bridging Innate and Adaptive Immunity

Dendritic cells are professional antigen-presenting cells found in tissues. They capture antigens, migrate to lymph nodes, and initiate adaptive immune responses by presenting antigens to naïve lymphocytes.

Organization of the Adaptive Immune Response

Adaptive immune responses are initiated in secondary lymphoid organs (lymph nodes, spleen, Peyer’s patches), where antigen-presenting cells stimulate naïve lymphocytes.

• Antigen presentation: Dendritic cells present antigens to T and B cells.

• Lymphocyte activation: Naïve lymphocytes are activated and proliferate.

• Lymphocyte programming: Cells differentiate into effector or memory cells.

Antigenic Specificity

The high specificity of the adaptive immune system is due to antigen-specific receptors:

• B cells: Immunoglobulins (antibodies)

• T cells: T cell receptors (TCRs)

• Each lymphocyte expresses a unique receptor, conferring one specificity per cell.

Clonal Selection Theory

Clonal selection is the process by which lymphocytes with receptors specific to an antigen are activated, proliferate, and differentiate into effector and memory cells.

• Only a small number of lymphocytes are specific to any given antigen.

• Upon antigen encounter, these cells expand and mount a targeted response.

Effector Functions of T Cells

• Helper T cells (CD4+): Direct immune responses, activate macrophages, stimulate B cells, and produce cytokines. Subtypes include Th1 (cell-mediated immunity), Th2 (humoral immunity), and Th17 (neutrophil responses).

• Cytotoxic T cells (CD8+): Kill infected or abnormal cells via perforin-granzyme and CD95 pathways.

Effector Functions of B Cells and Antibodies

B cells differentiate into plasma cells that secrete antibodies. Antibodies bind to epitopes and function in:

• Activation of complement and inflammation

• Neutralization

• Opsonization

• Agglutination

• Antibody-dependent cellular cytotoxicity (ADCC)

Classes of Antibodies

Antibodies are classified into five main types, each with distinct functions and properties.

Class	Structure	Function	Location
IgG	Monomer	Complement activation, opsonization, neutralization	Blood, extracellular fluid
IgM	Pentamer	First antibody produced, agglutination	Blood
IgA	Dimer	Mucosal immunity	Mucous membranes, secretions
IgE	Monomer	Allergic responses, defense against parasites	Bound to mast cells
IgD	Monomer	B cell receptor	B cell surface

T-Dependent Antibody Immunity

The induction of T-dependent antibody immunity involves four steps:

1. Antigen presentation for helper T cell activation and proliferation

2. Differentiation of helper T cells into Th2 cells

3. Activation of B cells

4. Proliferation and differentiation of B cells into plasma and memory cells

Immunological Memory

Memory cells are produced during the adaptive immune response and persist in lymphoid tissues.

• Primary response: Slow, produces small amounts of antibodies.

• Secondary response: Rapid and robust due to memory cells.

Types of Acquired Immunity

Acquired immunity can be classified as naturally or artificially acquired, and as active or passive.

	Active	Passive
Naturally Acquired	Body responds to antigens during infection	Antibodies transferred from mother to offspring
Artificially Acquired	Vaccination introduces antigens	Injection of preformed antibodies

Immune Pathology and Autoimmunity

Immune pathology refers to tissue damage caused by immune responses, often seen in autoimmune diseases such as rheumatoid arthritis, lupus, and type I diabetes.

• Immune responses may cause more harm than pathogens.

• Autoimmune diseases result from loss of self-tolerance.

Summary Table: Adaptive Immune Response Steps

Step	Description
Recognition	Detection of antigen by dendritic cells
Alarm	Activation and migration of antigen-presenting cells
Inflammation	Recruitment of immune cells to site of infection
Innate cell recruitment	Mobilization of innate immune cells
Innate effector function	Initial defense mechanisms
Adaptive response	Activation, proliferation, and differentiation of lymphocytes

Additional info: The adaptive immune system is essential for long-term protection and forms the basis for immunization strategies. Its dysfunction can lead to immunodeficiency or autoimmunity, highlighting the importance of specificity and regulation in immune responses.