Types of Immune Cells: Myeloid & Lymphoid Lineages

Overview of Immune Cells

• The discussion builds on previous information about immune cells and their functions.

Immune Cell Origins

• Most immune cells originate in the bone marrow from hematopoietic stem cells.

• Hematopoietic stem cells also produce:

  • Red blood cells

  • Platelets

• To become white blood cells, a stem cell can follow two differentiation tracks:

  • Myeloid lineage

  • Lymphoid lineage

Development of Immune Cells

• Hematopoietic stem cells differentiate into immune cells through specific molecular cues in the bone marrow.

• Historically, immune cells were characterized by their appearance under a microscope.

• Modern classification utilizes molecular tools based on surface proteins and gene expression.

Myeloid Derived Innate Immune Cells

Types of Myeloid Cells

• Key types include:

  • Macrophages

  • Specialize in killing pathogens through phagocytosis, a process where pathogens are engulfed and ingested.

  • Clean tissues by removing dead or dying cells.

  • Tissue resident macrophages originate from the fetal yolk sac and have a crucial role in homeostasis.

  • During inflammation, monocytes from the bloodstream can differentiate into macrophages.

  • Secrete cytokines that shape the immune response.

  • Granulocytes

  • Characterized by granules containing antimicrobial chemicals.

  • Types of granulocytes:

    • Neutrophils

    • Most abundant type in blood; first responders to infection.

    • Excellent at phagocytosis, especially of bacteria, but have a short lifespan.

    • Produce pus during infections.

    • Eosinophils

    • Less common, release cytokines promoting wound healing and tissue repair.

    • Basophils

    • Least common but largest granulocytes; secrete cytokines and signaling molecules.

    • Mast Cells

    • Similar to basophils but found in connective tissue; involved in allergic responses.

    • Play a role in targeting large parasites.

• Importance of granulocytes includes involvement in allergies and asthma.

Dendritic Cells

• Capable of phagocytosis but do not dominate pathogen clearance.

• Functions include:

  • Patrol tissues for infection signs.

  • Transport antigens to lymph nodes, activating T cells and linking innate and adaptive immune responses.

Innate Immune Cells from Lymphoid Lineage

• Innate lymphoid cells include:

  • Natural Killer (NK) Cells

  • Recognize and destroy cancer cells and virus-infected cells.

• Innate immune cells can respond to general features of pathogens but lack specificity for individual pathogens.

Adaptive Immune Cells

• Derived from lymphoid lineage; known as lymphocytes.

Major Types of Lymphocytes

1. B Cells

   • Function to produce antibodies, which are Y-shaped proteins that bind pathogens.

   • Antibodies block pathogens and mark them for destruction.

   • Require activation help from T cells.

2. T Cells

   • Three major types based on function:

     • Killer T Cells

     • Express CD8 and target cancerous or infected self-cells.

     • Helper T Cells

     • Express CD4; assist in activating B cells and guide the immune response through cytokines.

     • Regulatory T Cells

     • Also express CD4; secrete cytokines that moderate immune responses and can eliminate overactive killer T cells.

• T and B cells can persist long term in the body, leading to faster responses to previously encountered pathogens.

Summary of Immune System Functionality

• The immune system consists of billions of myeloid and lymphoid-derived white blood cells.

• These cells circulate in the bloodstream and patrolling tissues for damage or infection.

• They communicate via cytokines and chemokines to perform diverse functions geared towards destroying pathogens and healing tissues.

Future Topics

• The next topic will cover the lymphatic system, which is integral to immune function.

• Each immune cell type will receive detailed discussions in future tutorials.