Innate Immunity: Nonspecific Defenses of the Host

Innate Immunity

Innate immunity provides defenses against any pathogen; it is rapid and present at birth.

Activation of the Innate System

The innate immune system is activated through recognition of pathogen-associated molecular patterns (PAMPs) by toll-like receptors (TLRs) found on host cells. This activation induces the release of cytokines that regulate immune responses.

Second Line of Defense Components

Components of the second line of defense include various immune cells such as phagocytes (neutrophils, macrophages), inflammation processes, and the complement system.

Interferons

Interferons are cytokines produced by host cells (fibroblasts, lymphocytes, macrophages) that have antiviral activity. They are effective against viral infections by inducing nearby cells to produce antiviral proteins that inhibit viral replication.

Formed Elements

Formed elements are the cells and cell fragments suspended in plasma, including erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

Granulocytes and Their Roles

Granulocytes are a category of white blood cells characterized by the presence of granules in their cytoplasm. They include:

  • Neutrophils: Highly phagocytic, most active in early infection stages.

  • Basophils: Release histamine and play a role in inflammation.

  • Eosinophils: Phagocytic and toxic against parasites and helminths.

  • Mast cells: Release histamine and participate in inflammatory responses.

Agranulocytes and Their Roles

Agranulocytes are white blood cells without granules in their cytoplasm, including:

  • Monocytes: Mature into macrophages in tissues where they perform phagocytosis.

  • Dendritic cells: Found in skin and mucous membranes; they act as antigen-presenting cells.

  • Lymphocytes (T cells, B cells, NK cells): Vital for adaptive immunity as they recognize specific antigens.

Phagocytes

Phagocytes are cells that ingest and digest microorganisms and other substances. They include neutrophils, eosinophils, and macrophages.

Mechanism of Phagocytosis

Phagocytosis involves several key steps:

  1. Chemotaxis: Phagocytes are attracted to pathogens by chemical signals.

  2. Adherence: Phagocytes attach to the surface of the microorganism, facilitated by opsonization.

  3. Ingestion: Phagocytes extend pseudopods to engulf the microorganism.

  4. Digestion: The ingested microorganism is digested in a phagolysosome.

Process of Inflammation

Inflammation is a local defensive response triggered by tissue damage, characterized by:

  • Pain

  • Redness

  • Swelling (edema)

  • Heat
    Functions include destroying or limiting the injurious agent and repairing damaged tissue.

Diapedesis

Diapedesis is the process by which white blood cells move through the endothelium of blood vessels into surrounding tissues during inflammation, allowing them to reach sites of infection.

Development of Fever

Fever develops as a response to infection, typically initiated by pyrogens released from pathogens or immune cells, leading to an increase in body temperature as a defense mechanism.

Complement System

The complement system consists of over 30 serum proteins that enhance the immune response by promoting opsonization, inflammation, and cytolysis of pathogens.

Activation of the Complement System

Complement can be activated via three pathways:

  • Classical pathway: Triggered by antigen-antibody complexes.

  • Alternative pathway: Initiated directly by microbial surfaces.

  • Lectin pathway: Activated by carbohydrate-containing mannose on microbes.

Mechanism Differences

While all pathways result in complement activation, they differ in initiating triggers (antigen-antibody complex vs. microbial surfaces).

Outcomes of the Complement System

Outcomes include:

  • Cytolysis: Formation of the membrane attack complex (MAC).

  • Opsonization: Enhanced phagocyte attachment to microbes.

  • Inflammation: Release of histamine and other inflammatory cytokines from mast cells.

Role of Iron-Binding Proteins

Iron-binding proteins play a role in sequestering iron to limit its availability for pathogenic bacteria. Types include:

  • Transferrin: Found in blood and tissue fluids.

  • Lactoferrin: Found in milk, saliva, and mucus.

  • Ferritin: Found in the liver, spleen, and red bone marrow.

  • Hemoglobin: Located in red blood cells, binding oxygen and iron.