Immunology - Adaptive & Innate Immunity: Components & Responses

• Innate Immunity: Non-specific, immediate response (hours), lacks immunological memory, utilizes physical barriers, complement, and phagocytes.
• Adaptive Immunity: Highly specific, delayed initial response (days), forms immunological memory, utilizes B cells, T cells, and antibodies.

The most abundant circulating leukocytes; primary first responders to acute bacterial infection and inflammation. Function via phagocytosis, release of reactive oxygen species (ROS), degranulation, and formation of Neutrophil Extracellular Traps (NETs).

Tissue-resident mononuclear phagocytes derived from monocytes. Act as key antigen-presenting cells (APCs), secrete inflammatory cytokines (e.g., TNF-$\alpha$, IL-1, IL-6), and clear cellular debris and pathogens.

Innate lymphoid cells that identify and lyse virus-infected or transformed (tumor) cells without prior sensitization. Activated by the target cell's absence or downregulation of MHC Class I ("missing-self hypothesis").

A plasma protein cascade activated via three pathways:
1. Classical Pathway: Triggered by antigen-antibody complexes (C1q binding to IgG or IgM).
2. Alternative Pathway: Triggered by spontaneous C3 hydrolysis on microbial surfaces.
3. Lectin Pathway: Triggered by Mannose-Binding Lectin (MBL) binding to microbial carbohydrates.

• Opsonization: C3b targets pathogens for phagocytosis.
• Anaphylatoxins: C3a and C5a recruit leukocytes and induce mast cell degranulation.
• Membrane Attack Complex (MAC): C5b-C9 assembly forms structural pores in microbial cell membranes, inducing lysis.

Expressed on all nucleated cells. Presents endogenous intracellular antigens (e.g., viral or tumor proteins) to CD8+ Cytotoxic T cells via interaction with the ${\alpha }_3$ domain.

Expressed exclusively on professional Antigen-Presenting Cells (APCs) (dendritic cells, macrophages, B cells). Presents exogenous extracellular antigens (e.g., phagocytosed bacteria) to CD4+ Helper T cells.