Innate Immunity

Innate Immunity

The innate immune responses are the first line of defense against invading pathogens.

Epithelial Surfaces Help Prevent Infection

Human Cells Recognize Conserved Features of Pathogens.

Complement Activation Targets Pathogens for Phagocytosis or Lysis.

Toll-like Proteins Are an Ancient Family of Pattern Recognition Receptors.

Phagocytic Cells Seek, Engulf, and Destroy Pathogens.

Activated Macrophages Recruit Additional Phagocytic Cells to Sites of Infection.

Virus-Infected Cells Take Drastic Measures to Prevent Viral Replication

Natural Killer Cells Induce Virus-Infected Cells to Kill Themselves

In vertebrates, the skin and other epithelial surfaces, including those lining the lung and gut.

provide a physical barrier between the inside of the body and the outside world.

The interior epithelial surfaces are also covered with a mucus layer that protects these surfaces

contains substances that kill pathogens or inhibit their growth.

defensins

They constitute a diverse family with a broad spectrum of antimicrobial activity

Including the ability to kill or inactivate

Bacteria

Gram-negative

Gram-positive

parasites (including protozoa and nematodes)

fungi (including yeasts)

viruses like HIV

It is then up to the innate and adaptive immune systems to recognize and destroy them, without harming the host.

pattern recognition receptors that can be activated directly by pathogen-associated immunostimulants.

They were originally identified by their ability to amplify and “complement” the action of antibodies

belonging to three distinct pathways of complement activation

the classical pathway

the lectin pathway

the alternative pathway

act locally to activate C3

Is the pivotal component of complement

is a transmembrane protein

large extracellular domain

consisting of a series of leucine-rich repeats

Receptors responsible for triggering host cell gene expression in response to pathogens

macrophages

neutrophils

variety of cell-surface receptors that enable them to recognize and engulf pathogens.

Once the pathogen has been phagocytosed

The phagosome is acidified and fuses with lysosomes, which contain lysozyme and acid hydrolases that can degrade bacterial cell walls and proteins.

Many pathogens have developed strategies that allow them to avoid being ingested by phagocytes

The only unusual molecule associated with viruses is the double-stranded RNA (dsRNA)

intermediate in the life cycle of many viruses.

Host cells can detect the presence of dsRNA and initiate a program of drastic responses in attempt to eliminate it.

Another way that the interferons help defend themselves against viruses is by stimulating both innate and adaptive cellular immune responses.

Like cytotoxic T cells, NK cells destroy virus-infected cells by inducing the infected cell to kill itself by undergoing apoptosis.