Innate and Adaptive Body Defenses: The Immune System

The Immune System: Overview

Introduction to Body Defenses

The immune system protects the body from disease by recognizing and eliminating pathogens and abnormal cells. It is divided into two main branches: innate (nonspecific) defenses and adaptive (specific) defenses. These systems work together to provide comprehensive protection.

Innate (Nonspecific) Defenses

Surface Barriers

The first line of defense consists of physical and chemical barriers that prevent pathogen entry. These include the skin and mucous membranes, which act as a physical shield and secrete chemicals that inhibit or destroy microorganisms.

• Skin: Acts as a tough, keratinized barrier resistant to acids, bases, and toxins.

• Mucous membranes: Trap and expel pathogens; found in respiratory, digestive, urinary, and reproductive tracts.

• Protective chemicals: Acid mantle, enzymes (e.g., lysozyme), mucin, defensins, and other antimicrobial substances.

Internal Defenses

If pathogens breach surface barriers, the second line of defense is activated. This includes various cells and chemicals that target invaders.

• Phagocytes: White blood cells that ingest and digest foreign invaders.

• Natural Killer (NK) cells: Lymphocytes that target and kill virus-infected and cancerous cells.

• Inflammation: Localized response to injury or infection, characterized by redness, heat, swelling, and pain.

• Antimicrobial proteins: Interferons and complement proteins that attack pathogens directly or enhance other defenses.

• Fever: Systemic response that inhibits pathogen growth and speeds up tissue repair.

Phagocytosis

Phagocytosis is the process by which phagocytes engulf and destroy pathogens. The steps include recognition, engulfment, digestion, and expulsion of waste.

• Recognition and adherence to pathogen's surface molecules.

• Engulfment into a phagosome.

• Fusion with a lysosome to form a phagolysosome.

• Digestion by lysosomal enzymes.

• Exocytosis of indigestible material.

Inflammation

Inflammation is triggered by tissue injury and serves to contain damage, remove pathogens, and initiate repair. It involves the release of chemicals, increased blood flow, and recruitment of immune cells.

• Cardinal signs: Redness, heat, swelling, pain.

• Stages: Chemical release, vasodilation and increased permeability, phagocyte mobilization.

• Benefits: Prevents spread of infection, disposes of debris, alerts adaptive immunity, and sets the stage for healing.

Antimicrobial Proteins

Antimicrobial proteins enhance innate defenses by directly attacking pathogens or hindering their ability to reproduce.

• Interferons (IFNs): Proteins released by virus-infected cells to warn neighboring cells and activate immune cells.

• Complement system: A group of plasma proteins that, when activated, enhance inflammation, promote phagocytosis (opsonization), and cause cell lysis via the membrane attack complex (MAC).

Fever

Fever is a systemic response to infection, mediated by pyrogens released from leukocytes and macrophages. It increases body temperature, which can inhibit pathogen growth and enhance immune reactions.

• Promotes sequestration of iron and zinc, which bacteria need to multiply.

• Increases metabolic rate, speeding up tissue repair.

Adaptive (Specific) Defenses

Characteristics of Adaptive Immunity

The adaptive immune system targets specific pathogens and retains memory for faster responses upon re-exposure. It is divided into humoral and cellular branches.

• Specificity: Targets particular antigens.

• Systemic: Not restricted to the initial infection site.

• Memory: Mounts stronger attacks on previously encountered pathogens.

Antigens and Antigenic Determinants

Antigens are substances that provoke an immune response. They possess antigenic determinants (epitopes) that are recognized by antibodies and lymphocyte receptors.

• Complete antigens: Have immunogenicity and reactivity.

• Haptens: Small molecules that become immunogenic only when attached to body proteins.

• Self-antigens (MHC proteins): Mark body cells as "self"; important in immune recognition and organ transplantation.

Lymphocyte Development and Selection

Lymphocytes (B and T cells) undergo development, maturation, and selection to ensure they recognize foreign antigens but tolerate self-antigens.

• Origin: Both B and T cells originate in red bone marrow.

• Maturation: T cells mature in the thymus (undergo positive and negative selection); B cells mature in bone marrow.

• Positive selection: T cells must recognize self-MHC to survive.

• Negative selection: T cells that bind too strongly to self-antigens are eliminated to prevent autoimmunity.

Antigen-Presenting Cells (APCs)

APCs such as dendritic cells, macrophages, and B cells process and present antigens to T cells, initiating adaptive immune responses.

Humoral Immunity (B Cells and Antibodies)

Activation and Clonal Selection of B Cells

B cells are activated when their receptors bind to specific antigens. This triggers clonal selection, resulting in the formation of plasma cells (which secrete antibodies) and memory B cells.

Immunological Memory

The primary immune response is slow and weak, while the secondary response is faster and stronger due to memory B cells.

Antibody Structure and Classes

Antibodies (immunoglobulins) are Y-shaped proteins composed of two heavy and two light chains, with variable regions forming antigen-binding sites. There are five main classes: IgM, IgA, IgD, IgG, and IgE, each with distinct roles in immunity.

Mechanisms of Antibody Action

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

• Neutralization: Blocks harmful effects of toxins/viruses.

• Agglutination: Clumps cell-bound antigens.

• Precipitation: Clumps soluble antigens.

• Complement fixation: Leads to cell lysis.

Cellular Immunity (T Cells)

T Cell Activation and Differentiation

T cells are activated when they recognize antigen fragments presented by MHC proteins on APCs. CD4 cells become helper T cells, while CD8 cells become cytotoxic T cells. Activation requires antigen binding and co-stimulation.

• Helper T cells (TH): Activate B cells, other T cells, and macrophages; direct the immune response.

• Cytotoxic T cells (TC): Destroy virus-infected, cancerous, or foreign cells.

• Regulatory T cells (TReg): Suppress immune responses to prevent autoimmunity.

Clinical Considerations

Immune Disorders

• Immunodeficiency: Impaired immune function (e.g., SCID, AIDS).

• Autoimmune diseases: Immune system attacks self (e.g., rheumatoid arthritis, type 1 diabetes).

• Hypersensitivities: Overactive immune responses causing tissue damage (e.g., allergies).

Summary Table: Innate vs. Adaptive Defenses

Feature	Innate Defenses	Adaptive Defenses
Specificity	Nonspecific	Specific (targets unique antigens)
Memory	None	Has memory (faster, stronger response on re-exposure)
Main Components	Surface barriers, phagocytes, NK cells, inflammation, antimicrobial proteins, fever	B cells (antibodies), T cells
Response Time	Immediate	Slower (must be primed)