Chapter 16

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 antigen.

• Unresponsiveness to self: Prevents attack on the body's own cells.

• Memory: Remembers previous encounters for faster future responses.

Key Characteristics of Adaptive Immunity

Adaptive immunity is defined by four main features:

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

• Tolerance to self: Avoids attacking self-antigens, preventing autoimmune disorders.

• Minimal self-damage: Immune responses are regulated to minimize harm to host tissues.

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

Cellular Components of Adaptive Immunity

Leukocytes and Lymphocytes

Leukocytes, or white blood cells, are central to immune function. Among them, lymphocytes are the primary cells involved in adaptive immunity.

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

• T cells: Mature in thymus; include helper (CD4+) T cells (direct immune response via cytokines) and cytotoxic (CD8+) T cells (kill infected or cancerous cells).

• Natural Killer (NK) cells: Kill infected or cancerous cells as part of the innate response.

Dendritic Cells: Antigen Presenting Cells

Dendritic cells are professional antigen-presenting cells found in tissues. They capture antigens, migrate to lymph nodes, and initiate adaptive immune responses, serving as a bridge between innate and adaptive immunity.

Organization of the Adaptive Immune Response

Secondary Lymphoid Organs

Adaptive immune responses are initiated in secondary lymphoid organs such as lymph nodes, spleen, and Peyer's patches. These sites facilitate interactions between antigen-presenting cells and naïve lymphocytes.

Antigen Recognition and Clonal Selection

The high specificity of the adaptive immune system is due to antigen-specific receptors on B cells (immunoglobulins) and T cells (T cell receptors). Each lymphocyte expresses a unique receptor, conferring one specificity per cell.

• Clonal selection: Upon antigen encounter, only lymphocytes with receptors specific to the antigen are activated, proliferate, and differentiate into effector and memory cells.

Mechanisms of Adaptive Immune Response

Helper T Cells (CD4+)

Helper T cells are programmed during priming to produce specific cytokines, tailoring the immune response to the pathogen type.

• Th1 cells: Produce INFγ, activate macrophages for intracellular microbes (cell-mediated immunity).

• Th2 cells: Produce IL-4 and IL-5, stimulate antibody production and anti-parasite responses (humoral immunity).

• Th17 cells: Produce IL-17, stimulate neutrophil responses for extracellular bacteria and fungi.

Cytotoxic T Cells (CD8+)

Cytotoxic T cells kill infected or abnormal cells via two main pathways:

• Perforin-granzyme pathway: Releases proteins that induce apoptosis in target cells.

• CD95 pathway: Activates apoptotic enzymes through cell surface glycoproteins.

B Cells and Antibody Production

B cells produce antibodies that bind to specific epitopes on antigens. Antibodies function in several ways:

• Activation of complement and inflammation

• Neutralization

• Opsonization

• Agglutination

• Antibody-dependent cellular cytotoxicity (ADCC)

Antibody Classes

There are five main classes of antibodies, each with distinct structures and functions.

Class	Structure	Function	Location
IgG	Monomer	Complement activation, opsonization, neutralization	Blood, extracellular fluid
IgM	Pentamer	Primary response, agglutination	Blood
IgA	Dimer	Secretory immunity	Mucosal surfaces
IgE	Monomer	Allergic responses, anti-parasite	Bound to mast cells
IgD	Monomer	B cell receptor	B cell surface

Inducement of T-Dependent Antibody Immunity

Steps in T-Dependent Antibody Response

The T-dependent antibody response involves four main 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 and Secondary Response

Memory cells are produced during the primary immune response and persist in lymphoid tissues. They enable a much faster and more robust response upon subsequent exposure to the same antigen.

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

• Secondary response: Rapid, produces large amounts of antibodies.

Types of Acquired Immunity

Active vs Passive Immunity

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

	Active	Passive
Naturally Acquired	Body responds to antigens that enter naturally (e.g., infection)	Antibodies transferred from mother to offspring (placenta, breast milk)
Artificially Acquired	Antigens introduced in vaccines; body produces antibodies	Preformed antibodies introduced by injection

Immune Pathology and Tolerance

Immune Pathology

Immune responses can sometimes cause pathology, especially in autoimmune diseases. Examples include atherosclerosis, type I diabetes, rheumatoid arthritis, multiple sclerosis, lupus, asthma, inflammatory bowel disease, and possibly Alzheimer's disease.

• Immune pathology is often observed without clear understanding of initiation or antigen specificity.

Lymphocyte Development and Tolerance

Lymphocyte development occurs in bone marrow (B cells) and thymus (T cells). Tolerance mechanisms prevent immune responses against self-antigens, maintaining homeostasis and preventing autoimmunity.

Summary Table: Steps in Adaptive Immune Response

Step	Description
Recognition	Detection of antigen by dendritic cells and lymphocytes
Alarm	Activation signals sent to recruit immune cells
Inflammation	Local response to infection or injury
Innate cell recruitment	Mobilization of innate immune cells
Innate effector function	Initial defense mechanisms
Adaptive response	Activation, proliferation, and differentiation of antigen-specific lymphocytes

Conclusion

The adaptive immune system is a complex, highly regulated network that provides specific, inducible, and long-lasting protection against pathogens. Its ability to distinguish self from non-self, generate memory, and tailor responses to different types of invaders is fundamental to health and disease prevention.

Additional info:

Some diagrams and tables were inferred and expanded for clarity and completeness based on standard immunology textbook content.