Study Notes: The Immune System – Innate and Adaptive Defenses

The Immune System

Innate and Adaptive Defenses

The immune system protects the body from pathogens through two main types of defense: innate (nonspecific) and adaptive (specific) immunity. Understanding the differences and examples of each is essential for grasping how the body responds to infection.

• Innate Defenses: These are present from birth and provide immediate, nonspecific protection against pathogens. Examples include physical barriers (skin, mucous membranes), phagocytic cells (neutrophils, macrophages), natural killer cells, and antimicrobial proteins.

• Adaptive Defenses: These develop after exposure to specific antigens and provide targeted, long-lasting immunity. Key components are B cells (produce antibodies) and T cells (mediate cellular immunity).

• Example: The skin acts as a physical barrier (innate), while antibodies produced by B cells target specific pathogens (adaptive).

Cardinal Signs of Acute Inflammation

Acute inflammation is the body's immediate response to injury or infection, characterized by several classic signs.

• Redness (Rubor): Increased blood flow to the affected area.

• Heat (Calor): Elevated temperature due to increased blood flow.

• Swelling (Tumor): Accumulation of fluid (edema).

• Pain (Dolor): Release of chemicals that stimulate nerve endings.

• Loss of function (Functio laesa): Sometimes included as a fifth sign.

• Example: A cut on the skin becomes red, swollen, warm, and painful due to inflammation.

Two Arms of Adaptive Immunity and Associated Cell Processes

Adaptive immunity consists of two main branches, each involving specific cell types and mechanisms.

• Humoral Immunity: Mediated by B cells and antibodies. Effective against extracellular pathogens.

• Cell-Mediated Immunity: Mediated by T cells (CD4+ helper T cells and CD8+ cytotoxic T cells). Effective against intracellular pathogens.

• Example: Antibodies neutralize bacteria in the bloodstream (humoral), while cytotoxic T cells destroy virus-infected cells (cell-mediated).

Antigen Presenting Cells (APCs)

APCs are specialized cells that process and present antigens to T cells, initiating the adaptive immune response.

• Major APCs: Dendritic cells, macrophages, and B cells.

• Function: Display antigen fragments on their surface using MHC molecules to activate T cells.

• Example: Dendritic cells capture antigens and migrate to lymph nodes to activate T cells.

Classes of Antibodies (Immunoglobulins)

Antibodies are proteins produced by B cells that bind to specific antigens. There are several classes, each with unique functions.

Class	Main Function
IgG	Most abundant; provides long-term immunity and crosses the placenta.
IgM	First antibody produced; effective in agglutination and complement activation.
IgA	Found in mucosal areas; protects body surfaces.
IgE	Involved in allergic reactions and defense against parasites.
IgD	Functions mainly as a B cell receptor.

CD4+/CD8+ and Effector T Cells

T cells are critical for cell-mediated immunity and are classified based on surface markers and function.

• CD4+ T Cells (Helper T Cells): Activate other immune cells by releasing cytokines.

• CD8+ T Cells (Cytotoxic T Cells): Destroy infected or abnormal cells.

• Effector T Cells: Activated T cells that carry out immune responses, such as killing infected cells or helping B cells produce antibodies.

• Example: CD4+ T cells stimulate B cells to produce antibodies, while CD8+ T cells kill virus-infected cells.

Additional info: The above notes expand on brief points by providing definitions, examples, and a summary table of antibody classes for clarity and completeness.