Adaptive Immune System: Innate and Adaptive Body Defenses

Adaptive Immune System: Innate and Adaptive Body Defenses

Overview of Adaptive Defenses

The adaptive immune system is a highly specific defense mechanism that eliminates pathogens and abnormal cells. It amplifies the inflammatory response and activates complement proteins. Unlike innate immunity, adaptive immunity must be primed by exposure to a specific foreign substance, which takes time.

• Specificity: Targets specific antigens.

• Systemic: Not restricted to the initial site of infection.

• Memory: Mounts stronger attacks upon subsequent exposures to the same antigen.

• Main Branches:

  1. Humoral (antibody-mediated) immunity

  2. Cellular (cell-mediated) immunity

Humoral Immunity

Humoral immunity involves antibodies produced by lymphocytes that circulate in body fluids. These antibodies bind temporarily to target cells, inactivate them, and mark them for destruction by phagocytes or complement. Humoral immunity targets extracellular pathogens.

Cellular Immunity

Cellular immunity involves lymphocytes acting directly or indirectly against target cells. They kill infected cells or release chemicals that enhance the inflammatory response and activate other immune cells. Cellular immunity targets intracellular pathogens and abnormal cells.

Antigens

Definition and Characteristics

Antigens are substances that can mobilize adaptive defenses and provoke an immune response. They are the targets of all adaptive immune responses and are usually large, complex molecules not normally found in the body (nonself).

• Can be complete antigens or haptens (incomplete antigens)

• Contain antigenic determinants

• Can be self-antigens

Complete Antigens and Haptens

Complete antigens have immunogenicity (ability to stimulate proliferation of specific lymphocytes) and reactivity (ability to react with activated lymphocytes and antibodies). Examples include foreign proteins, polysaccharides, lipids, and nucleic acids.

Haptens are small molecules that are not immunogenic by themselves but may become immunogenic if they attach to the body's own proteins, leading to harmful immune responses.

• Examples: poison ivy, animal dander, detergents, cosmetics

Antigenic Determinants

Antigenic determinants are specific parts of an antigen that antibodies or lymphocyte receptors bind to. Most antigens have several determinants, mobilizing different lymphocyte populations and forming various antibodies.

Self-Antigens: MHC Proteins

Self-antigens are proteins on cell surfaces that are not antigenic to self but may be antigenic to others. Major histocompatibility complex (MHC) proteins are glycoproteins unique to each individual, presenting self or foreign antigens to T lymphocytes.

Lymphocytes and Antigen-Presenting Cells

Lymphocyte Development, Maturation, and Activation

T and B lymphocytes share common developmental steps:

1. Origin: Both originate in red bone marrow.

2. Maturation: T cells mature in the thymus; B cells mature in bone marrow. They develop immunocompetence (recognize one specific antigen) and self-tolerance (unresponsive to own antigens).

3. Seeding Secondary Lymphoid Organs: Naive, immunocompetent lymphocytes colonize secondary lymphoid organs (lymph nodes, spleen).

4. Antigen Encounter and Activation: First encounter with antigen triggers clonal selection and differentiation into active cells.

5. Proliferation and Differentiation: Activated lymphocytes proliferate, forming clones. Most become effector cells; some become memory cells.

Lymphocyte Education in the Thymus

T cells undergo positive and negative selection in the thymus:

• Positive selection: T cells must recognize self-MHC proteins; those that fail are destroyed.

• Negative selection: T cells that bind to self-antigens are destroyed, ensuring self-tolerance.

Antigen-Presenting Cells (APCs)

APCs engulf antigens and present fragments to T cells. Major types include dendritic cells, macrophages, and B cells.

• Dendritic cells: Found in connective tissues and epidermis; most effective antigen presenters.

• Macrophages: Widely distributed; activate T cells and trigger inflammatory responses.

• B cells: Present antigens to helper T cells for their own activation.

Overview of B and T Lymphocytes

B and T lymphocytes have distinct roles in immunity. The following table summarizes their characteristics:

Type of Immune Response	B Lymphocytes	T Lymphocytes
Type	Humoral	Cellular
Antibody Secretion	Yes	No
Primary Targets	Extracellular pathogens	Intracellular pathogens
Site of Origin	Red bone marrow	Red bone marrow
Site of Maturation	Red bone marrow	Thymus
Effector Cells	Plasma cells	Cytotoxic T cells, Helper T cells, Regulatory T cells
Memory Cell Formation	Yes	Yes

Humoral Immune Response

Activation and Differentiation of B Cells

B cells are activated when antigens bind to their surface receptors, triggering clonal selection, proliferation, and differentiation into plasma cells (antibody-secreting effector cells) or memory cells.

Immunological Memory

Primary immune response occurs upon first exposure to an antigen, with a lag period of 3–6 days and peak antibody levels in 10 days. Secondary response is faster, stronger, and longer-lasting due to memory cells.

Active and Passive Humoral Immunity

• Active immunity: B cells encounter antigens and produce antibodies. Can be naturally acquired (infection) or artificially acquired (vaccination).

• Passive immunity: Ready-made antibodies are introduced. Can be naturally acquired (maternal antibodies) or artificially acquired (injection of antibodies).

Antibodies (Immunoglobulins)

Structure and Classes

Antibodies are proteins secreted by plasma cells, capable of binding specifically to antigens. They consist of four polypeptide chains (two heavy, two light) with variable and constant regions. The constant region determines antibody class and function.

Immunoglobulin Classes

• IgM: First antibody released; potent agglutinating agent; fixes complement.

• IgA: Found in mucus and secretions; prevents pathogen entry.

• IgD: Functions as B cell receptor.

• IgG: Most abundant; crosses placenta; secondary response.

• IgE: Active in allergies and parasitic infections; triggers histamine release.

Antibody Targets and Functions

Antibodies inactivate and tag antigens for destruction, forming antigen-antibody complexes. Defensive mechanisms include:

• Neutralization: Blocks harmful sites on pathogens.

• Agglutination: Clumps cells together.

• Precipitation: Cross-links soluble molecules.

• Complement fixation: Triggers cell lysis and enhances inflammation.

Clinical Application: Parasitic Infections

Parasitic worms require different immune strategies. IgE antibodies bind to the worm's surface, marking it for destruction by eosinophils, which release toxic contents to lyse the parasite.

Monoclonal Antibodies

Monoclonal antibodies are pure antibodies specific for a single antigenic determinant, produced by hybridomas. They are used in research, clinical testing, and cancer treatment.

Summary of Antibody Actions

Antigen-antibody complexes prepare antigens for destruction by innate defenses. Antibodies target extracellular pathogens and can act intracellularly if attached to viruses before cell entry.

Additional info: This study guide covers the adaptive immune system, focusing on humoral and cellular immunity, antigen structure, lymphocyte development, and antibody function, as outlined in Chapter 21 of Human Anatomy & Physiology.