Immune System Part 2: 1-29

Immune System: Adaptive Defenses

Overview of Adaptive (Specific) Immunity

The adaptive immune system is a specialized defense mechanism that protects the body against infectious agents and abnormal cells. Unlike innate immunity, adaptive immunity is highly specific, systemic, and possesses memory, allowing for stronger responses upon re-exposure to known antigens.

• Specificity: Targets specific antigens through specialized receptors.

• Systemic Action: Not limited to the initial site of infection; acts throughout the body.

• Memory: Remembers previous encounters, resulting in faster and stronger responses upon subsequent exposures.

• Amplifies Inflammation: Enhances the inflammatory response to pathogens.

• Activates Complement: Stimulates the complement system to aid in pathogen destruction.

• Priming Required: Must be activated by initial exposure to a specific foreign substance; this process takes time.

Arms of Adaptive Immunity

Adaptive immunity consists of two overlapping components:

• Humoral (Antibody-Mediated) Immunity: Involves antibodies produced by B lymphocytes that circulate in body fluids and target extracellular pathogens.

• Cellular (Cell-Mediated) Immunity: Involves T lymphocytes that directly attack infected or abnormal cells, or indirectly regulate immune responses.

Humoral Immunity

Mechanism and Function

Humoral immunity is mediated by antibodies produced by B lymphocytes. These antibodies circulate freely in body fluids and bind to antigens on pathogens, marking them for destruction.

• Antibody Binding: Antibodies attach temporarily to target cells.

• Inactivation: Antibodies can neutralize pathogens, preventing their activity.

• Opsonization: Mark pathogens for destruction by phagocytes or complement proteins.

• Extracellular Targets: Humoral immunity is most effective against pathogens located outside cells (e.g., bacteria, viruses in extracellular fluid).

Example: When a person is vaccinated, B cells produce antibodies that circulate and provide protection against future infections by the same pathogen.

Cellular Immunity

Mechanism and Function

Cellular immunity is mediated by T lymphocytes, which act against infected or abnormal cells through direct and indirect mechanisms.

• Direct Action: Cytotoxic T cells kill infected or cancerous cells.

• Indirect Action: Helper T cells release cytokines that enhance the inflammatory response and activate other immune cells, such as macrophages.

• Cellular Targets: Cellular immunity is most effective against intracellular pathogens (e.g., viruses inside cells) and abnormal cells (e.g., cancer cells).

Example: During a viral infection, cytotoxic T cells recognize and destroy virus-infected cells, preventing the spread of the virus.

Key Terms and Concepts

Antigen

• Definition: An antigen is any substance that can provoke an immune response by being recognized as foreign by the body.

• Types: Complete antigens (fully immunogenic) and haptens (incomplete antigens that become immunogenic when attached to host proteins).

• Antigenic Determinants: Specific regions on an antigen that are recognized and bound by antibodies or lymphocyte receptors.

Immunocompetence

• Definition: The ability of a lymphocyte to recognize and bind to a specific antigen.

• Development: B cells become immunocompetent in the bone marrow; T cells in the thymus.

Self-Tolerance

• Definition: The ability of the immune system to avoid attacking the body's own cells and antigens.

• Role of HLA/MHC: Human Leukocyte Antigens (HLA), also known as Major Histocompatibility Complex (MHC) proteins, help the immune system distinguish self from non-self.

Comparison of Humoral and Cellular Immunity

Feature	Humoral Immunity	Cellular Immunity
Key Cells	B lymphocytes	T lymphocytes
Effector Molecules	Antibodies	Cytokines, cytotoxic molecules
Targets	Extracellular pathogens	Intracellular pathogens, abnormal cells
Mechanism	Neutralization, opsonization, complement activation	Direct cell killing, regulation of immune responses

Summary of Steps in the Immune Response

1. Origin: All lymphocytes originate in the red bone marrow.

2. Maturation: B cells mature in the bone marrow; T cells mature in the thymus.

3. Seeding: Immunocompetent but naive lymphocytes migrate to secondary lymphoid organs (e.g., lymph nodes, spleen).

4. Antigen Encounter and Activation: Lymphocytes encounter antigens and are activated (clonal selection).

5. Proliferation and Differentiation: Activated lymphocytes proliferate and differentiate into effector and memory cells.

Additional info:

• Immunological memory is a hallmark of adaptive immunity, allowing for rapid and robust responses upon re-exposure to the same antigen.

• Antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells play a crucial role in activating T cells by presenting antigen fragments.