Understanding Phagocytosis and Immune Responses

Lymphocyte Migration and Immune Response

Lymphocytes constantly migrate through the blood and lymph, moving from one lymphoid organ to another. This process:

• Enhances the chance that an antibody will encounter its specific antigen.
• The spleen filters blood, while other lymphoid organs filter lymph.

Phagocytosis: A Non-Specific Innate Immune Response

Phagocytosis is the ingestion of microorganisms or particulate matter by a cell. It is performed by phagocytes, which are certain types of white blood cells or their derivatives.

Actions of Phagocytic Cells

1. Among the granulocytes, neutrophils are the most important phagocytes.
2. Enlarged monocytes transform into wandering macrophages and fixed macrophages.
3. Fixed macrophages are located in selected tissues and are part of the mononuclear phagocytic system.
4. Granulocytes predominate during the early stages of infection, whereas monocytes predominate as the infection subsides.

Mechanism of Phagocytosis

1. Chemotaxis: Phagocytes are attracted to microorganisms by microbial products, damaged cells, cytokines, and peptides from the complement system.
2. Adhesion: Toll-like receptors on a phagocyte adhere to Pathogen-Associated Molecular Patterns (PAMPs) on the microbial cells. Adherence may be facilitated by opsonization, which involves coating the microbe with serum proteins like complement or antibodies.
3. Ingestion: Phagocytes extend pseudopods to engulf microorganisms via endocytosis.
4. Digestion:
   • The phagosome fuses with a lysosome.
   • This forms a phagolysosome. Lysosomal enzymes and reactive oxygen intermediates degrade bacterial components.
5. Killing: An oxidative burst occurs, leading to lysis after excessive cell damage.
6. Release: Debris exits the phagocyte by exocytosis.

Microbial Evasion of Phagocytosis

1. Many phagocytized microorganisms are killed by lysosomal enzymes and oxidizing agents.
2. However, some microbes are not killed by phagocytes and can even reproduce within them.
3. Evasion mechanisms include M protein, capsules, leukocidins, and the prevention of phagolysosome formation.

Inflammation: A Response to Cell Damage

Inflammation is a bodily response to cell damage. It is characterized by redness, pain, heat, swelling, and sometimes the loss of function. Tumor Necrosis Factor (TNF), released by host cells, stimulates the inflammatory response.