Natural Resistance - Cellular Factors of Natural Resistance
Cellular factors are key components of the non-specific (innate) immune system. They provide the body's first cellular response against pathogens in natural immunity and play a crucial role alongside humoral factors. These cells respond through direct interactions with pathogens or by recruiting other immune cells.
The innate immune system includes cellular defences, primarily consisting of phagocytic leukocytes (such as neutrophils and macrophages) and specialised lymphocytes (Natural Killer cells).
Key cellular factors involved in natural resistance discussed in the sources include:
- Phagocytes:
- Perform phagocytosis, the process of engulfing, absorbing, and destroying pathogens.
- Include Macrophages and Neutrophils (also known as polymorphonuclear neutrophilic granulocytes or microphages). Eosinophils are also listed as professional phagocytes. Dendritic cells are also listed under phagocytes in the innate system.
- Macrophages are considered the most effective phagocytes and can move outside the vascular system into tissues.
- Neutrophils and macrophages play key roles in phagocytosing and killing fungi. They can secrete cytokines to recruit other immune cells.
- Phagocytes exhibit ameboid motility and are attracted to infection sites by chemotactic substances, such as the complement components C3a and C5a.
- Phagocytic killing mechanisms involve both oxygen-dependent processes (like the oxidative burst producing reactive oxygen radicals) and oxygen-independent mechanisms (including low pH, lysozyme, and defensins).
- Natural Killer (NK) Cells:
- Are specialised lymphocytes that are part of the innate immune system.
- They provide a rapid response to viral infections and tumor cells. They are considered a first line of defense against viral infections.
- NK cells are distinct from T and B lymphocytes.
- They have a unique ability to recognize stressed cells in the absence of antibodies and MHC molecules. They identify virus-infected cells by
- recognizing changes in MHC class I expression (which viruses often downregulate).
- Upon recognition, NK cells kill target cells by releasing perforin and granzymes, inducing apoptosis (programmed cell death).
- They can produce IFN-gamma (IFNc) very early following activation. Virus-induced IFNa and IFNb promote NK activation.
- They do not require a specific receptor for activation.
- NK cells play a role in Antibody-Dependent Cellular Cytotoxicity (ADCC).
- Other cells:
- Mast cells and Basophils are listed as components of the nonspecific (innate) immune system. Basophils contribute to degranulation induced by C3a and C5a.