Introduction
This topic covers the protective mechanisms of the immune system in infectious diseases: innate and adaptive defence against bacteria, viruses and fungi (mycoses). Innate immunity is important in the first hours and days of infection; acquired immunity plays the crucial role in recovery from severe infections. The balance between the infectious agent and the host immune response determines the outcome.
Immune response in bacterial infections
Innate immunity
Innate immunity is the first line of defence, acting rapidly in the initial hours and days of infection.
- Physical barriers: the skin and mucous membranes prevent bacteria from entering the body. The skin has an acidic environment (pH 3-5) and secretes antimicrobial proteins such as lysozyme and psoriasin. Mechanical processes also expel bacteria: ciliary action in the respiratory tract, peristalsis in the digestive tract and urine flow in the urogenital tract.
- Humoral factors (chemical defences):
- The complement system is a biochemical cascade that helps get rid of pathogens. Activation via the classical, alternative or lectin pathways can lead to opsonization of bacteria (coating them for phagocytosis) and 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 and is effective against Gram-positive bacteria. It is found in tears, saliva, sweat, mucus and breast milk.
- Interferons (IFNs) are cytokines important in viral infections, but type II IFN (IFN-γ) activates macrophages and NK cells and is relevant in defence against some bacteria.
- Other factors: acute phase proteins such as C-reactive protein (CRP), which aid pathogen recognition and opsonization, transferrin and lactoferrin.
- Cellular factors:
- Phagocytes (neutrophils and macrophages) are crucial. Neutrophils are often the first responders, engulfing pathogens and killing them with mechanisms including reactive oxygen species (ROS).
- Macrophages engulf bacteria and present bacterial antigens to T cells. They are central to granuloma formation in infections such as tuberculosis.
- Natural killer (NK) cells defend against intracellular pathogens, including some bacteria, and can produce IFN-γ.
- Inflammation and fever: inflammation is a critical initial defence, 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 raising body temperature.
- Normal flora (microbiota) on skin and mucous membranes contributes to non-specific defence by competing with pathogens for resources and space.