Who Does What at Each Layer | AI Learning Guide

Look at the comparison table. The barrier layer is made of epithelial cells, mucus, cilia, and antimicrobial peptides — no immune cells are required for it to work. The innate effector layer adds complement proteins, phagocytes, natural killer cells, and cytokines. Complement C3b is the opsonin that coats microbes; the membrane attack complex punches holes in bacterial membranes. Phagocytes engulf and kill. The antigen presentation layer is where dendritic cells carry fragments to lymph nodes and display them on MHC class II. That step is the bridge — without it, the adaptive layer never gets started. The adaptive layer has CD4+ helper T cells, CD8+ cytotoxic T cells, B cells, plasma cells, and antibodies, with regulatory T cells holding the response in check. The important point is that each layer has a specific job. If you remove a cell type or a molecule, you remove a specific capability, and the infections that follow tell you which capability was lost. That is the logic we will use in every later chapter.

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Cells and effector molecules by layer

Barrier

Epithelial cells with tight junctions
Mucus and cilia for clearance
Defensins and low pH

Innate effectors

Complement: C3b, membrane attack complex
Phagocytes: neutrophils, macrophages
Natural killer cells
Cytokines: IL-1, TNF, IL-6

Antigen presentation

Dendritic cells
Macrophages and B cells
MHC class II with peptide

Adaptive effectors

CD4+ helper T cells
CD8+ cytotoxic T cells
B cells and plasma cells
Antibodies: IgM, IgG, IgA, IgE
Regulatory T cells

The division of labor is not just a list of parts. Each layer covers a specific gap. Barriers stop most organisms without any immune activation. Complement and phagocytes handle organisms that breach the barrier and do so within minutes. Antigen presentation converts a local innate encounter into a systemic adaptive response. Antibodies neutralize extracellular pathogens and enhance phagocytosis, while cytotoxic T cells eliminate cells already infected. Regulatory T cells prevent the adaptive response from continuing after the threat is gone. Because each capability is assigned to a defined set of cells and molecules, a defect in one layer produces a predictable loss of function.

To locate any immune failure, identify which layer's cells or effector molecules are missing or dysfunctional. The infection pattern will reflect the gap that layer normally covered.