Leukocyte Pathogen Recognition Figure (Figure 17.20). Reprinted from OpenStax Microbiology (p. 711), by N. Parker, M. Shneegurt, A.-h. Thi Tu, B. M. Forester, and P. Lister, 2016, Rice University. Copyright 2016 by Rice University.

Leukocyte Pathogen Recognition Figure

Molecular structures/motifs common to many pathogens

Pathogen-associated Molecular Patterns (PAMPs)

Structures that allow phagocytes to detect PAMPs. PRRs on macrophages can also detect cell distress signals to mount inflammatory responses against injuries and other diseases. Ex.: Toll-like receptors

Pattern Recognition Receptors (PRRs)

While pathogen opsonzation can help phagocytes in recognition, phagocytes can also recognize pathogen-associated molecular patterns (PAMPs) (non-opsonin dependent), which are vital to microbe structure. Pattern recognition receptors (PRRs) in phagocytes interact with PAMPs, which acts as the first step in pathogen recognition as well as signals phagocytic activation.

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The steps of leukocyte defense against pathogens are:
1. Extravasation (Diapedesis)
2. Pathogen Recognition
3. Pathogen Degradation

Steps of Leukocyte Immune Response

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The process in which leukocytes are able to move across capillaries within tissues. Initiated by complement factor C5a and proinflammatory and chemotactic cytokines released by nearby macrophages and tissues. These components promote the expression of cellular adhesion molecules, which bind to leukocytes and lead to transendothelial migration. Leukocytes then exit the bloodstream and reach the infected areas to begin their pathogen attack. Some leukocytes respond faster than others, with neutrophils responding within hours and monocytes taking up to days.

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Leukocyte: Extravasation (Diapedesis)

Leukocyte: Pathogen Recognition

After pathogen recognition and binding, the pathogen is put into a vesicle and brought into a phagocyte (phagocytosis). After the phagocyte is engulfed, lysosomal enzymes digest the pathogen. Reactive oxygen species from respiratory bursts and reactive nitrogen compounds can also degrade pathogens. Waste leftover from degradation is excreted as well as processed by leukocytes. This processing allows key antigens to be presented on the cell surface for adaptive immune responses.

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