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)

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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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.

Leukocyte: Pathogen Degradation

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

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Formed elements involved in the innate and adaptive immune response

White Blood Cells (Leukocytes)

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Subdivisions of leukocytes include:

- Granulocytes
- Agranulocytes

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