Human Body's Immune System

• The body has three layers of defense: epithelia and skin, the innate immune system, and the adaptive immune system.

Barrier Immunity

• Physical barriers that prevent pathogens from entering the organism.
• Produces antimicrobial proteins to kill invading organisms.
• Exists in respiratory, gastrointestinal, genitourinary systems, and nose/pharynx linings.

Innate Immune System

• Present from birth and immediately activated upon pathogen attack.
• General or nonspecific, targeting anything identified as foreign.
• Activated by the presence of antigens and their chemical properties.

Types of Cells (Leukocytes/WBCs)

• Includes neutrophils, eosinophils, basophils, mast cells, macrophages, monocytes, dendritic cells, and natural killer cells.

Neutrophils

• Most abundant immune cells (60-70%).
• Phagocytes that kill and digest pathogens using enzymes.
• Found in the bloodstream, short-lived, highly mobile, and first responders to infection/inflammation.

Eosinophils

• 1-3% of WBCs with granules containing chemicals and enzymes.
• Fight multicellular parasites and some bacterial infections.
• Responsible for tissue damage and inflammation in diseases like asthma.

Basophils

• Least common WBCs (0.5-1%).
• Fight parasitic infections and play a role in blood clotting.
• Involved in various allergic reactions.

Mast Cells

• Reside in connective tissue and mucous membranes.
• Involved in wound healing, microbial defense, and serious allergic reactions.

Macrophages

• Phagocytic cells that migrate from blood to tissue spaces to engulf invaders.
• Large eaters

Monocytes

• Circulate in the blood with a single-lobed nucleus.

Dendritic Cells

• Located in tissues exposed to the external environment.
• Antigen-presenting cells that defend against viruses.
• "Dendrite" means branch

Natural Killer Cells

• Do not attack microbes directly.
• Kill defective or infected cells, such as tumor cells or virally infected cells.

Adaptive Immune System

• Not present at birth; activated by pathogen exposure.
• Uses immunological memory to learn and enhance response.
• Slower to respond than the innate immune system.
• Lymphocytes (B cells and T cells) derived from hematopoietic stem cells in bone marrow.

Cellular Immunity

• T cells destroy infected cells.
• Steps:
  • Exposure to a pathogen.
  • Pathogen infects a body cell; antigens displayed on the infected cell.
  • Phagocyte engulfs and displays pathogen antigens, activating an inactive T cell.
  • T cell divides into memory T cells and active (plasma) T cells.
  • Active T cells bind to and destroy the infected cell by releasing proteins and enzymes.

B Lymphocyte (B Cell)

• Production and maturation occur in the bone marrow.
• Released into the blood and recirculate through lymph, blood, and secondary lymphoid tissues.

Humoral Immunity

• B cells produce antibodies.
• Steps:
  • Exposure to a pathogen.
  • B cell membrane-bound antibodies bind the pathogen.
  • B cell engulfs and displays pathogen antigens.
  • Helper T cell binds to antigen-presenting B cell.
  • B cell divides into memory B cells and active B cells.
  • Active B cells produce antibodies to attack the pathogen.

Antibody

• A protein produced by the immune system to bind specifically to a target.
• Circulates in the bloodstream throughout the body.

Antibodies and Antigens

• Antibodies target parts of pathogens.
• Antigens are substances/parts of pathogens that generate an immune response, leading to specific antibody production.

Five Major Types of Antibodies

• IgM, IgG, IgA, IgE, IgD.

IgM

• First antibody produced; coats the pathogen and promotes endocytosis.

IgG

• The major antibody produced; activates other parts of the immune response, leading to neutralization and destruction.

IgA

• Important for mucosal immune response; prevents pathogens from crossing the epithelium.

IgE

• Activates mast cells, producing histamine, associated with allergic reactions.

IgD

• The role of this antibody is still unclear at this point.