Chapter 16: Adaptive Immunity – Specific Defense Mechanisms

Adaptive Immunity

Overview of Adaptive Immunity

Adaptive immunity, also known as specific immunity, is the branch of the immune system that targets particular pathogens and foreign molecules (antigens). Unlike innate immunity, adaptive immunity develops over time and possesses immunological memory, allowing for a more rapid and effective response upon subsequent exposures to the same antigen.

• Specificity: Targets specific antigens.

• Memory: Remembers previous encounters, leading to faster responses.

• Components: B cells, T cells, lymphatic system, bone marrow, thymus.

Types of Immune Responses

Humoral Response

The humoral response involves the production of antibodies (immunoglobulins) by B lymphocytes. These antibodies bind to antigens, tagging them for destruction by other immune cells.

• B cells: Produce antibodies in response to antigens.

• Antibodies: Proteins that specifically bind to antigens.

• Function: Neutralize pathogens, opsonize for phagocytosis, activate complement system.

Cell-Mediated Response

The cell-mediated response is carried out by T lymphocytes, which recognize antigens presented on the surface of infected or abnormal cells and directly destroy these cells.

• T cells: Recognize and destroy infected or foreign cells.

• Autoimmune Disorders: Malfunctions in cell-mediated immunity can lead to the immune system attacking the body's own cells.

Antigens

Definition and Types of Antigens

Antigens are molecules recognized as foreign by the immune system, provoking a specific immune response. They can originate from pathogens, environmental sources, or even the body's own cells.

• Exogenous Antigens: Enter the body from outside (e.g., bacteria, viruses).

• Endogenous Antigens: Generated within cells due to infection or mutation.

• Autoantigens: Normal body molecules mistakenly targeted by the immune system.

• Examples: Bacterial endotoxins, viral proteins, fungal and protozoan proteins, food, dust.

Antibody Structure and Function

Antibody (Immunoglobulin) Functions

Antibodies are Y-shaped proteins produced by B cells. They play several roles in immune defense:

• Neutralization: Bind to toxins or pathogens, preventing their activity.

• Opsonization: Coat pathogens to enhance phagocytosis.

• Complement Activation: Trigger the complement cascade, leading to pathogen lysis.

• Agglutination: Clump pathogens together for easier clearance.

• Antibody-Dependent Cellular Cytotoxicity (ADCC): Recruit other immune cells to destroy tagged cells.

Cells Producing Antibodies: B lymphocytes (plasma cells).

Classes of Antibodies

There are five major classes of antibodies, each with distinct functions and properties:

Class	Main Function	Location
IgG	Major antibody in blood; provides long-term immunity	Blood, extracellular fluid
IgM	First antibody produced in response; effective in agglutination	Blood
IgA	Protects mucosal surfaces	Mucous membranes, saliva, tears
IgE	Involved in allergic responses and defense against parasites	Blood, tissues
IgD	Functions mainly as a B cell receptor	Surface of B cells

Antigen Processing and Presentation

Major Histocompatibility Complex (MHC)

The MHC is a group of proteins on cell surfaces that present antigens to T cells, enabling recognition and response.

• MHC Class I: Presents endogenous antigens to cytotoxic T cells.

• MHC Class II: Presents exogenous antigens to helper T cells.

Antigen Processing

Antigen processing involves breaking down antigens and presenting them on MHC molecules for recognition by T cells.

• Exogenous Pathway: Antigens are taken up, processed, and presented on MHC II.

• Endogenous Pathway: Antigens from within the cell are presented on MHC I.

Primary and Secondary Immune Responses

Primary Response

The primary immune response occurs upon first exposure to an antigen. B cells produce antibodies, and some become memory cells.

• Characteristics: Slower, lower antibody levels.

• Memory Cells: Survive in lymph nodes for years.

Secondary (Memory) Response

The secondary response is triggered by re-exposure to the same antigen. Memory cells rapidly proliferate and produce large amounts of antibody.

• Characteristics: Faster, higher antibody levels, more effective.

Acquired Immunity

Types of Acquired Immunity

Acquired immunity can be classified based on how immunity is obtained and whether the body produces its own antibodies.

Type	Source	Active/Passive	Description
Naturally Acquired Active	Exposure to pathogens in daily life	Active	Body produces its own antibodies
Naturally Acquired Passive	Maternal antibodies (e.g., via placenta or breast milk)	Passive	Antibodies received from another individual
Artificially Acquired Active	Vaccination	Active	Body produces antibodies in response to vaccine
Artificially Acquired Passive	Injection of antibodies (e.g., antiserum)	Passive	Antibodies received from another individual

Summary Table: Comparison of Types of Acquired Immunity

Type	How Acquired	Antibody Source	Example
Naturally Acquired Active	Infection	Self	Recovering from chickenpox
Naturally Acquired Passive	Maternal transfer	Mother	Antibodies in breast milk
Artificially Acquired Active	Vaccination	Self	Flu vaccine
Artificially Acquired Passive	Injection	Donor	Rabies antiserum

Key Terms and Definitions

• Antibody (Immunoglobulin): Protein produced by B cells that binds to antigens.

• Antigen: Molecule recognized as foreign by the immune system.

• B cell: Lymphocyte responsible for antibody production.

• T cell: Lymphocyte responsible for cell-mediated immunity.

• Memory cell: Long-lived lymphocyte that responds rapidly upon re-exposure to antigen.

• Major Histocompatibility Complex (MHC): Proteins presenting antigens to T cells.

• Autoimmune disorder: Condition where the immune system attacks self-antigens.

Relevant Equations

While adaptive immunity is primarily conceptual, the following equation represents antibody concentration over time during primary and secondary responses:

$\text{Antibody concentration} = \text{Initial response} + \text{Memory response}$

For example, the secondary response can be modeled as:

$\text{Secondary response} = \text{Memory cell activation} \times \text{Antibody production rate}$

Example Application

Vaccination: When a person receives a vaccine (artificially acquired active immunity), their B cells produce antibodies and memory cells. Upon later exposure to the pathogen, the immune system mounts a rapid and effective secondary response, preventing disease.

Additional info: Academic context was added to clarify antibody classes, immune response mechanisms, and examples of acquired immunity.