Innate Immunity against Bacterial Infections
Innate immunity provides the first line of defence, acting rapidly in the initial hours and days of infection. Key components include:
- Physical Barriers:
- The skin and mucous membranes act as physical barriers preventing bacteria from entering the body. The skin has an acidic environment (pH 3-5) and secretes antimicrobial proteins like lysozyme and psoriasin.
- Mechanical processes like ciliary action in the respiratory tract, peristalsis in the digestive tract, and urine flow in the urogenital tract also help expel bacteria.
- Humoral Factors (Chemical Defences):
- The complement system is a biochemical cascade that helps get rid of pathogens. Activation via classical, alternative, or lectin pathways can lead to opsonization of bacteria (coating them for phagocytosis) and the formation of Membrane Attack Complexes (MACs) that can lyse bacteria, particularly Gram-negative species. Complement can be activated directly by pathogens or via antibodies.
- Lysozyme breaks down the peptidoglycan layer of bacterial cell walls, effective against Gram-positive bacteria. It is found in various body fluids like tears, saliva, sweat, mucus, and breast milk.
- Interferons (IFNs) are cytokines important in the immune response to viral infections, but Type II IFN (IFNγ) is important for activating immune cells like macrophages and NK cells, relevant in defence against some bacteria.
- Other factors include acute phase proteins like C-reactive protein (CRP) which aid in pathogen recognition and opsonization, transferrin, and lactoferrin.
- Cellular Factors:
- Phagocytes such as Neutrophils and Macrophages are crucial. Neutrophils are often the first responders, engulfing pathogens and killing them using mechanisms including reactive oxygen species (ROS).
- Macrophages also play a critical role, engulfing bacteria and presenting bacterial antigens to T-cells. They are central to granuloma formation in infections like tuberculosis.
- Natural Killer (NK) cells are part of innate immunity and involved in defence against intracellular pathogens, including some bacteria, and can produce IFNγ.
- Inflammation and Fever:
- Inflammation is a critical initial defence mechanism, mediated by cytokines and inflammatory molecules. It promotes vasodilation and attracts immune cells to the infection site.
- Fever, induced by pyrogens, helps inhibit bacterial growth by increasing body temperature.
- Normal Flora (Microbiota):
- The normal human flora on skin and mucous membranes contributes to non-specific defence by competing with pathogens for resources and space.