Principles of Innate Immunity

Study Notes

Innate immunity consists of physical and chemical barriers that prevent infections.
Lysozyme is effective at degrading bacterial cell walls.
Defensins disrupt the integrity of microbial membranes.
Antibodies create a link between adaptive and innate immunity.
Multiple layers of immune defense include barriers, innate immune cells, and adaptive immune cells, each with distinct functions.

Barrier tissues protect against microbial colonization through mechanical and chemical defenses.
Native flora compete with and inhibit pathogenic flora.
Antimicrobial agents, like lysozyme and defensins, provide broad-spectrum protection against infections.

The complement system includes pathways that converge on the cleavage of C3, initiating various immune responses like inflammation, phagocytosis, and membrane attack.
The classical pathway commences with C1, where C1q binds to antibodies, leading to enzymatic activation.

The classical pathway is primarily triggered by antibodies, while the lectin pathway is activated by carbohydrates on pathogen surfaces, and the alternative pathway can initiate spontaneously.
All pathways generate a C3 convertase, critical for further antimicrobial activity.

C3 cleavage activates inflammation, opsonization, and the formation of the membrane attack complex (MAC).
C3b acts as an opsonin, marking pathogens for phagocytosis and facilitating the clearance of immune complexes.
C5 convertase formation leads to MAC assembly, ultimately resulting in the lysis of target cells.

Phagocytic receptors identify pathogen-associated molecular patterns (PAMPs) and opsonins, enabling efficient pathogen recognition and engulfment.
Phagocytosis helps present antigens from external pathogens and clears dead cell debris.
Apoptosis, which can be triggered by infection, sequesters DAMPs (damage-associated molecular patterns) and can enhance inflammatory responses.
Necroptosis, a programmed form of necrosis, releases DAMPs and is also immunogenic.

Phagocytosis connects innate and adaptive immune responses through antigen presentation.
Phagocytes utilize harsh environments within phagolysosomes and cytoplasmic granules for antimicrobial actions.
Programmed cell death limits pathogen replication and modulates immune responses. The immunogenicity of DAMPs can influence inflammation, depending on their release or sequestration.