Innate Immune System

What is the innate immune system?

The immune system in place at birth. It is non-specific and responds immediately to an offense.

It includes:

• physical and physiological barriers

• antimicrobial molecules

• sentinel/phagocytic cells

Physical barriers of the innate immune system

• epithelium

  • tight junctions

  • cell turnover

• mucus/tears/sebum

  • pH

  • lysozyme

  • bile

  • pancreatic enzymes

• flushing/peristalsis

• commensal flora

Antimicrobial molecules

• produced by epithelial cells and leukocytes

• include defensins and cathelicidins

• directly toxic to bacteria, fungi, protozoa, enveloped viruses, tumor cells

• activate and recruit leukocytes

• bind and neutralize lipopolysaccharide

• stimulate wound healing and vascularization 

Opsonins

complement proteins and mucosal antibodies that bind to microbes and make them susceptible to phagocytosis

Sentinel cells

Cells that recognize pathogens early on in the innate immune system

• mast cells - initiate inflammation

• macrophages - inflammation and phagocytosis

• dendritic cells - process antigen to start adaptive response 

Hematopoiesis

• the process in which blood cells are created in the bone marrow (flat bones and proximal end of long bones in adults)

• creation of myeloid, erythroid, lymphoid, and platelet progenitors 

When do macrophages become activated/differentiate?

Monocytes circulate in the blood stream and differentiate into macrophages once they enter the tissue. 

Where do B cells mature?

• Bursa of Fabricius in birds

• Bone marrow in mammals

Where do T cells mature?

In the thymus

Neutrophils

• 2-5 segments of the nucleus

• clear/neutral stained cytoplasm

• majority of circulating leukocytes in most species

• primary function is to kill infecting microbes

  • phagocytosis

  • degranulation

    • some granules result in respiratory burst, bind iron, etc. 

  • neutrophil extracellular traps (NETs)

Eosinophils

• short transit time in blood

• morphologic differences in shape of granules between species

• cytoplasmic granules stain pink 

• functions

  • parasitic killing

  • participate in hypersensitivity reactions 

  • promote inflammation 

Basophils

• very rare in most species

• cytoplasmic granules stain blue/purple

• functions similar to eosinophils and mast cells

  • hypersensitivity reactions

  • participate in coagulation

  • rejection of parasites

Monocytes

• largest of the blood leukocytes

• circulating precursor to macrophages and dendritic cells

  • enter tissue to differentiate

• blue-gray cytoplasm with vacuoles 

Macrophages

• respond to inflammation

• phagocytosis of cell debris, foreign material, infectious agents

• secrete inflammatory cytokines to recruit other inflammatory cells 

• pale, basophilic, vacuolated cytoplasm 

Dendritic cells

• reside in connective and lymphoid tissues

• round nucleus, clear cytoplasm

• functions

  • sentinel cell

  • endo/phagocytosis

  • antigen presentation

Mast cells

• fine magenta to basophilic granules

• functions

  • vasodilation

  • vascular permeability

  • recruit other leukocytes

  • allergic responses

Natural Killer cells

• lymphoid lineage, but participates in innate immunity 

• first line of defense against viruses, some tumors

• secrete cytotoxic granules that drive apoptosis 

What cells in an aspirate would give you clues on if an infection is acute or chronic?

• neutrophils - acute

• macrophages/lymphocytes/plasma cells = chronic 

• both = chronic - active