Human Immunology Overview

The Skin

• The skin is the first line of defense against pathogens
• Sebum, produced by sebaceous glands, contains antimicrobial elements that kill harmful microbes
• A symbiotic relationship exists between the skin and microbes, where they co-exist in a balanced environment

Mucous Membranes

• Mucous membranes line hollow organs like the nose, mouth, and trachea
• Cilia, hair-like structures, move mucus containing foreign substances upward, preventing them from entering the lungs
• When mucociliary clearance is insufficient, coughing assists in removing particles from the lungs and trachea

Bone Marrow and Stem Cells

• The bone marrow is the birthplace of many immune cells, also known as leukocytes
• Stem cells divide asymmetrically or symmetrically to form daughter cells
• Symmetric cell division produces two stem cells or two progenitor cells
• Progenitor cells divide repeatedly, resulting in a large number of immune cells

Neutrophils

• Neutrophils are common immune cells that circulate in the bloodstream
• They belong to the innate immune system and migrate through blood vessels during inflammation
• Diapedesis is the process of neutrophils moving through the endothelial cells of blood vessels
• Chemotaxis guides neutrophils to infection sites by following cytokine signals

Phagocytosis

• Neutrophils engulf and destroy bacteria through a process called phagocytosis
• Some neutrophils release cytokines that attract other granulocytes and macrophages

Macrophages

• Macrophages arrive at infection sites before neutrophils
• They phagocytose bacteria and produce tumor necrosis factor, a cytokine with the following effects:
  • Suppresses hunger
  • Causes fever
  • Stimulates leukocyte production

T Cells

• T cells have receptors that recognize specific antigens presented by macrophages or professional antigen-presenting cells
• T cells originate in the bone marrow and mature in the thymus, hence the name "T" for thymus cells
• Mature T cells migrate to secondary lymphoid organs such as lymph nodes and spleen

Antigen Presentation

• Dendritic cells, professional antigen-presenting cells, present antigens to naive T cells in secondary lymphoid organs
• When a T cell encounters its specific antigen presented by a dendritic cell, it becomes activated
• Activation triggers clonal expansion, creating thousands of antigen-specific T cells

Cytotoxic T Cells

• Cytotoxic T cells scan other cells for antigens and initiate apoptosis, or programmed cell death, in infected cells
• Apoptosis ensures that pathogens trapped within infected cells are eliminated
• Macrophages eventually engulf the cellular debris from apoptosis

T Helper Cells

• T helper cells assist B cells during immune responses
• They provide necessary signals for the activation and maturation of B cells

B Cells

• B cells possess receptors for specific antigens, just like T cells
• They also originate in the bone marrow and circulate in the lymphatic system and bloodstream
• When a B cell encounters its specific antigen, it activates or migrates to lymphoid tissue to await T helper cell activation

Antibody Production

• Activated B cells undergo clonal expansion, producing thousands of new B cells, plasma blasts, and plasma cells
• Plasma cells produce antibodies, also known as immunoglobulins, which are released into the bloodstream
• Antibodies bind to antigens, neutralizing toxins and marking pathogens for easier macrophage recognition and phagocytosis

Opsonization

• Opsonization is the process by which antibodies mark pathogens for efficient phagocytosis by macrophages

Natural Killer Cells

• Natural killer cells are a type of lymphocyte that kill infected or cancerous cells without the need for antigen recognition

Platelets

• Platelets, also known as thrombocytes, are essential for blood clotting
• They originate in the bone marrow from megakaryocytes
• Megakaryocytes release platelets into circulation through their pseudopodia
• One megakaryocyte can produce thousands of platelets

Hemostasis

• Hemostasis is the process of blood clotting when a blood vessel is injured
• Platelets adhere to collagen fibers in the connective tissue, forming a platelet plug
• Fibrin strands form on platelets, further strengthening the clot and binding additional platelets and red blood cells