Innate Immunity: Nonspecific Defenses of the Host

Physical and Chemical Barriers

Innate immunity provides the first line of defense against pathogens, utilizing physical and chemical barriers to prevent infection.

• Skin and mucous membranes: Act as physical barriers to block pathogen entry.

• Cilia and mucous: Trap and remove microbes from respiratory passages.

• Complement proteins: Found in perspiration, contribute to microbial defense.

• Serving as prebiotics: Some mucous membranes support beneficial bacteria, aiding in defense.

Example: The skin prevents microbial invasion, while mucous membranes trap pathogens in the respiratory and digestive tracts.

Phagocytosis

Phagocytosis is a key process in innate immunity, involving the ingestion of solid material by a eukaryotic cell.

• Definition: The process by which phagocytes engulf and digest microbes and debris.

• Capsules: Some bacteria, such as Streptococcus pyogenes, have capsules that make them "slippery" to phagocytes, helping them evade immune detection.

• Inflammatory response: Tissue damage triggers inflammation, characterized by redness, swelling, heat, and pain.

Example: The ankle becomes red, swollen, and warm after injury due to the inflammatory response.

Adaptive Immunity: Specific Defenses of the Host

Overview of Adaptive Immunity

Adaptive immunity is the body's second and third line of defense, providing specific responses to pathogens through humoral and cellular mechanisms.

• Humoral immunity: Involves B cells and the production of antibodies.

• Cellular immunity: Involves T cells, including helper T cells and cytotoxic T cells.

• Memory cells: Allow for faster and stronger responses upon subsequent exposure to the same antigen.

Example: Memory B cells and plasma cells are produced after activation of B cells, leading to long-term immunity.

Antibody Structure and Function

Antibodies are proteins produced by B cells that bind to specific antigens, neutralizing or marking them for destruction.

• Variable region: The part of the antibody that binds the antigen.

• Classes of antibodies: IgM and IgG are found in serum; IgG lasts longer in the body.

• Functions: Agglutination, opsonization, and neutralization of pathogens.

Example: Agglutination and opsonization enhance phagocytosis by marking pathogens for ingestion.

Cell-Mediated Immunity

Cell-mediated immunity involves T cells that recognize and destroy infected host cells.

• Helper T cells (Th cells): Activate B cells and cytotoxic T lymphocytes.

• Cytotoxic T cells (CTLs): Kill infected cells by recognizing antigens presented on MHC I molecules.

• Recognition: CD4 glycoprotein is used by helper T cells to recognize infected host cells.

Example: Cytotoxic T cells destroy virus-infected cells by binding to MHC I-antigen complexes.

Clonal Selection and Expansion

Clonal selection is the process by which specific B or T cells are activated and proliferate in response to an antigen.

• Activation: BCR (B cell receptor) recognizes antigen, leading to clonal expansion.

• Plasma cells: Differentiated B cells that produce antibodies.

• Memory cells: Provide long-term immunity by storing information about specific antigens.

Example: Upon re-exposure to an antigen, memory cells rapidly produce antibodies.

Major Histocompatibility Complex (MHC)

MHC molecules present antigen fragments on the surface of cells, allowing T cells to recognize infected or abnormal cells.

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

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

Example: Dendritic cells present antigens via MHC II to activate helper T cells.

Immunological Memory and Secondary Response

Primary vs. Secondary Immune Response

The immune system responds more rapidly and effectively upon second exposure to an antigen due to memory cells.

• Primary response: Slow rise in antibody levels, followed by a gradual decline.

• Secondary response: Faster and stronger production of antibodies, especially IgG.

Example: IgG production peaks around day five in the secondary response.

Table: Comparison of Innate and Adaptive Immunity

Key Terms and Definitions

• Phagocytosis: Ingestion of microbes or particles by a cell.

• Antibody: Protein produced by B cells that binds antigens.

• Antigen: Substance that triggers an immune response.

• Helper T cell: T lymphocyte that activates other immune cells.

• Cytotoxic T cell: T lymphocyte that kills infected cells.

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

• MHC: Molecules on cell surfaces that present antigen fragments to T cells.

Relevant Equations

• Rate of antibody production:

• Clonal expansion:

Additional info: Equations are simplified representations of immune cell proliferation and antibody production rates.