Immune System: Innate and Adaptive Responses

• The immune system comprises organs, cells, and molecules that collaboratively generate an immune response, protecting against microorganisms like viruses and bacteria.
• The immune system has two main branches: the innate and adaptive immune responses.

Innate Immune Response

• Non-specific cells distinguish invaders from human cells without differentiating between invaders.
• It is a fast-acting response with no memory, responding identically to a pathogen upon repeated exposure.

Adaptive Immune Response

• Specific to viruses and bacteria, cells recognize unique pathogen parts (antigens) via receptors to mount targeted responses.
• It can recognize unlimited specific antigens and develop a specific defense against each.
• This response relies on cell activation and differentiation, requiring a few weeks to take effect.
• It has memory, allowing for stronger and faster responses to subsequent exposures.

White Blood Cells

• Myeloid cells are part of the innate response and include basophils, eosinophils, neutrophils, and macrophages (monocytes).

Phagocytic Cells

• These cells bind to pathogens, internalize them in a phagosome, which fuses with lysosomes to destroy the pathogen.
• Neutrophils, macrophages, and dendritic cells are all phagocytic and release cytokines.

Neutrophils

• Neutrophils primarily ingest and destroy pathogens through phagocytosis.
• During phagocytosis, pathogens are placed in a phagosome.
• Neutrophil destruction methods include:
  • Granule fusion with the phagosome to form a phagolysosome, lowering the pH and killing 2% of pathogens.
  • Oxidative burst: neutrophils produce highly reactive oxygen species (hydrogen peroxide) to destroy proteins and nucleic acids, eliminating the pathogen.

Dendritic Cells

• Dendritic cells originate from monocytes and ingest pathogens to present antigens to other immune cells, especially T cells.
• Dendritic cells are antigen-presenting cells found in skin and GI mucosa, areas exposed to external antigens.
• The process involves phagocytosis, breaking proteins into short amino acid chains (antigens), moving to the nearest lymph node, and presenting the antigen to T cells.
• Dendritic cells are the only cells that travel from skin/GI mucosa to lymph nodes.

Antigen-Presenting Cells

• T cells can only recognize antigens when presented on a Major Histocompatibility Complex (MHC).
• Antigen-presenting cells load antigens onto MHC molecules and display them to T cells for recognition and binding.

Lymphocytes

• Lymphocytes include B cells, T cells, and natural killer cells.
  • B and T cells = adaptive immune response.
  • Natural killer cells = innate immune response.
• Maturation sites:
  • Bone marrow: B cells and natural killer cells.
  • Thymus: T cells.

Natural Killer Cells

• Natural killer cells target cells infected with intracellular organisms (viruses) and cancer cells.
• Natural killer cells use several mechanisms to kill target cells:
  • Release cytotoxic granules that create holes in the target cell's membrane.
  • Release molecules that induce apoptosis in target cells.

B Cells

• B cells convert to plasma cells and produce antibodies by finding a corresponding antigen and presenting it to a T cell, which then triggers the transformation.
• B cells are also antigen-presenting cells, presenting to T cells, and are highly specific.
• Receptors bind only to matching antigens and can bind directly to an antigen without MHC molecule presentation.
• When a B cell binds to an antigen on a pathogen, it internalizes and degrades the antigen, then loads it onto an MHC molecule for presentation to T cells.
• T cell activation triggers B cell maturation into a plasma cell, which secretes antibodies against that specific pathogen.
• Antibodies have the same antigen specificity as the B cell, circulating in plasma and "tagging" pathogens for destruction.
• In humoral immunity, antibodies float freely in the blood.

T Cells

• Several key points to remember about T-cells: There are two main types: helper T-cells (CD4) and cytotoxic T-cells (CD8).
• T cells mature and function when antigen-presenting cells present antigens to them.
• Cell-mediated immunity is antigen-specific and involves differentiation from a naive T cell to a mature T cell.
• Naive T cells attach to antigen-presenting cells, usually dendritic cells.
• CD4 (helper) cells recognize antigens presented on MHC II molecules only and secrete cytokines to coordinate other immune cells’ actions.
• CD8 (cytotoxic) cells recognize antigens presented on MHC I molecules only and kill target cells carrying the matching antigen.

Innate Immune Response to a Pathogen

• Macrophages ingest bacteria and release cytokines.
• Cytokines make blood vessels leaky and attract eosinophils, basophils, and mast cells to the infection site.
• Neutrophils from the blood arrive to phagocytize and destroy pathogens, which often results in the neutrophil's destruction.
• Immature dendritic cells digest pathogens in tissues and travel to the lymph node to present antigens on MHC II proteins to naive T cells.
• Dendritic cells connect the innate and adaptive immune systems

Adaptive Immune Response to a Pathogen

• Bacteria entering lymph nodes encounter B cells.
• Specific B cells phagocytose the bacteria and present it to naive CD4+ T cells.
• Viruses inside host cells express viral antigens on MHC I molecules.
• Cytotoxic T cells recognize the antigen on the MHC I molecule and kill the infected cell.
• After interacting with its specific antigen, the T cell undergoes differentiation and clonal expansion.
• CD4+ T cells then induce B cells to become plasma cells that secrete antibodies.

Explore the immune system, including innate and adaptive immune responses. Learn how these systems protect against microorganisms, with a focus on the roles and functions of white blood cells in immunity. Discover the differences between the fast-acting innate response and the targeted adaptive response.