Adaptive Immunity: Specific Defenses of the Host (Chapter 17 Study Notes)

Adaptive Immunity: Specific Defenses of the Host

Introduction to Immunity

Adaptive immunity refers to the body's ability to specifically counteract foreign organisms or substances, known as antigens, through specialized lymphocytes and antibodies. This system is acquired through infection or vaccination and is distinguished by its specificity and memory.

• Immunity: The ability to specifically counteract foreign antigens.

• Antigen: Any substance that provokes an immune response.

• Antibody: Proteins produced in response to antigens.

• Acquired Immunity: Immunity developed during an individual's lifetime, either actively (exposure to antigen) or passively (transfer of antibodies).

Key Terminology

• Innate (nonspecific) immunity: Defenses against any pathogen; genetically inherited.

• Adaptive (specific) immunity: Induced response to a specific microbial invader or foreign substance.

• Serology: Study of reactions between antibodies and antigens.

• Antiserum: Blood-derived fluids containing antibodies.

• Globulins: Serum proteins, including gamma globulin (contains antibodies).

Separation of Serum Proteins by Gel Electrophoresis

Serum proteins can be separated by gel electrophoresis, which distinguishes globulins and albumin based on their migration toward the anode.

Protein Fraction	Migration
Gamma (γ) globulins	Closest to cathode
Beta (β) globulins	Intermediate
Alpha (α) globulins	Intermediate
Albumin	Closest to anode

Cytokines: Chemical Messengers of Immune Cells

Cytokines are chemical messengers produced by immune cells in response to stimuli, facilitating communication and coordination of immune responses.

• Interleukins: Communicate between leukocytes.

• Chemokines: Attract leukocytes to infection sites.

• Interferons (IFNs): Interfere with viral infections.

• Tumor Necrosis Factor (TNF): Involved in inflammation and autoimmune diseases.

• Hematopoietic cytokines: Regulate development of blood cells.

• Cytokine storm: Overproduction can cause harmful inflammation.

The Third Line of Defense

The adaptive immune system recognizes and responds specifically to foreign substances, distinguishing itself from innate immunity by its specificity and memory.

• Antigens: Substances that provoke a specific immune response.

• Immune system components: T cells, B cells, antibodies, and the lymphatic system.

Antigens and Antibody Generators

Nature and Types of Antigens

Antigens, also called immunogens, provoke highly specific immune responses. Most antigens are proteins or large polysaccharides, but lipids and nucleic acids can be antigenic when combined with proteins or polysaccharides.

• Microbial antigens: Capsules, cell walls, flagella, fimbriae, toxins, viral coats.

• Non-microbial antigens: Pollens, egg white, blood cell surface molecules, serum proteins, transplanted tissue molecules.

• Epitope (antigenic determinant): Specific region of an antigen recognized by antibodies.

• Haptens: Small molecules that are antigenic only when attached to carrier molecules.

Lymphocytes and Lymphoid Organs

Lymphocytes originate from stem cells in the bone marrow and seed primary lymphoid organs (bone marrow and thymus), where they differentiate into B and T cells.

• B cells: Mature in bone marrow; responsible for humoral immunity.

• T cells: Mature in thymus; responsible for cellular immunity.

Functions of Specific Immune Response

• Recognition: Identifies non-self substances.

• Response: Mobilizes defense molecules and cells to neutralize or destroy foreign material.

• Memory: Enables rapid and intense response upon subsequent exposures.

Types of Acquired Immunity

Active and Passive Immunity

Acquired immunity can be active (exposure to antigen) or passive (transfer of antibodies).

Type	Natural	Artificial
Active	Infection	Vaccination
Passive	Transplacental, colostrum	Injection of antibodies

Passive immunity is short-lived due to the short half-life of antibodies and lack of immunological memory.

Types of Adaptive Immunity

Acquisition	Active	Passive
Natural	Antigens enter body naturally; body produces antibodies and lymphocytes	Antibodies pass from mother to fetus via placenta or to infant via milk
Artificial	Antigens introduced in vaccines; body produces antibodies and lymphocytes	Preformed antibodies in immune serum introduced by injection

The Dual Nature of the Adaptive Immune System

Humoral vs. Cellular Immunity

The adaptive immune system consists of two arms: humoral (antibody-mediated) and cellular (cell-mediated) immunity.

• Humoral immunity: B cells produce antibodies that bind bacteria, toxins, and extracellular viruses for destruction.

• Cellular immunity: T cells directly attack infected or abnormal cells and regulate macrophage activity.

B Lymphocytes (B Cells)

B cells provide humoral immunity by recognizing antigens and releasing antibodies into blood and lymph. Upon activation, B cells proliferate into memory cells and plasma cells, the latter producing large quantities of antibodies.

• Memory cells: Retain information for rapid response upon re-exposure.

• Plasma cells: Secrete antibodies at high rates.

Antibodies (Immunoglobulins)

Antibodies are Y-shaped globular proteins with two identical light chains and two identical heavy chains. The variable region binds epitopes, while the constant (Fc) region determines the antibody class and can attach to host cells or complement.

• Valence: Number of antigen-binding sites (bivalent = 2).

• Five classes: IgG, IgM, IgA, IgD, IgE.

Immunoglobulin Structure

Antibodies are classified based on their constant region. The antigen-binding site is formed by the variable regions of the light and heavy chains.

Classes of Immunoglobulins

Class	Structure	Serum %	Location	Function	Half-life
IgG	Monomer	80%	Blood, lymph, intestine	Crosses placenta, enhances phagocytosis, neutralizes toxins/viruses	23 days
IgM	Pentamer	5-10%	Blood, lymph, B cells	First produced, agglutinates microbes, activates complement	5 days
IgA	Dimer/Monomer	10-15%	Mucous membranes, saliva, tears, milk	Prevents microbial attachment	6 days
IgD	Monomer	0.2%	Blood, lymph, B cells	Antigen receptor on B cells	3 days
IgE	Monomer	0.002%	Mast cells, basophils, blood	Allergic reactions, lysis of parasitic worms	2 days

Differentiation of T Cells and B Cells

Stem cells in bone marrow or fetal liver differentiate into B cells (in bone marrow) and T cells (in thymus), which then migrate to lymphoid tissues.

Clonal Selection

B cells are activated when antigens bind to their surface receptors, leading to proliferation into plasma cells (which secrete antibodies) and memory cells. Self-reactive B and T cells are destroyed during development (clonal deletion).

Self-Tolerance

• The immune system does not produce antibodies against self-antigens.

• Clonal deletion: Destruction of self-reactive B and T cells.

Antigen-Antibody Binding and Its Results

Antigen-antibody complexes form when antibodies bind antigens, with the strength of the bond termed affinity. These complexes protect the host by various mechanisms:

• Agglutination: Clumping of antigens for easier removal.

• Optimization: Coating antigens to enhance phagocytosis.

• Antibody-dependent cell-mediated cytotoxicity: Targeting large pathogens for destruction.

• Neutralization: Blocking toxins or viruses from binding to host cells.

• Activation of complement system: Triggering lysis of pathogens.

Summary Table: Protective Mechanisms of Antibody Binding

Mechanism	Description
Agglutination	Antibodies cause antigens to clump together, facilitating phagocytosis.
Opsonization	Antibodies coat antigens, enhancing their uptake by phagocytes.
Neutralization	Antibodies block attachment of toxins or viruses to host cells.
Complement Activation	Antibody binding triggers complement cascade, leading to lysis.
Antibody-dependent cell-mediated cytotoxicity	Antibodies recruit immune cells to destroy large pathogens.

Additional info: These notes cover the foundational concepts of adaptive immunity, including the nature of antigens and antibodies, the structure and function of immunoglobulins, and the mechanisms by which the immune system recognizes and responds to pathogens. The tables summarize key classifications and mechanisms for efficient exam review.