Goals and Types of Adaptive Responses
The development of an infectious process is always accompanied by the activation of the patient's defense mechanisms. These reactions can be innate, adaptive, or manifest as allergic or autoimmune processes.
Globally, the macroorganism pursues three goals:
- To recognize the invading pathogen.
- To eliminate the pathogen and clear it from the body.
- To repair structural and functional damage caused by the disease.
Classification of Defense Mechanisms
To prevent the development of an infection, the body employs two echelons of defense that work in close synergy, enhancing overall efficacy:
- Nonspecific factors. This is universal defense that triggers upon contact with any foreign agent. It includes mechanical barriers, reflex reactions, phagocytosis, leukocyte activity, and the bactericidal properties of biological fluids.
- Specific (immunogenic) factors. Targeted at combating a specific, previously recognized microorganism. They include cellular and humoral immune responses.
First Line of Defense: Barriers and Secretions
Many pathogens (e.g., in contact infections) can penetrate inside only if epithelial integrity is compromised. Healthy skin protects us through a dense stratum corneum and continuous desquamation of the epithelium, which mechanically removes bacteria.
Inside the body, distinct anatomical and histological barriers operate: ciliated epithelium in the bronchi, the brush border in the intestine, as well as histohematic barriers (including the blood-brain barrier) and cell membranes.
The chemical properties of secretions play a massive role:
- Secretory immunoglobulins (especially IgA). They block bacterial ligands, preventing them from attaching to epithelial receptors. Clinical fact: newborns initially have low levels of secretory IgA, making them highly vulnerable to bacterial infections.
- Lysozyme and glycoproteins (IgM).
- Low pH. Lactic acid from sweat and fatty acids on the skin, as well as hydrochloric acid in the stomach, create a lethal environment for microbes. The stomach is virtually the only part of the GI tract where living bacteria are normally absent.
Role of P-Glycoprotein
This is a crucial factor that prevents bacterial toxins from massively penetrating mucous membranes. It functions as a cellular pump, expelling biological substrates from cells back into the organ lumen.
In the small intestine, it limits the absorption of toxins; in the liver and kidneys, it accelerates their excretion with bile and urine; and in barrier tissues, it blocks their deeper penetration. Experiments show that in genetic defects (gene knockout) of P-glycoprotein in mice, susceptibility to colitis increases dramatically.
Cellular Defense and Phagocytosis
If the barriers are breached, leukocytes (mononuclear cells and granulocytes, primarily neutrophils) step in. They destroy the enemy directly or by releasing special substances known as leukokinins.
- Phagocytosis is the engulfment and intracellular destruction of the microbe. Main phagocytic cells:
- Blood neutrophils and monocytes.
- Tissue macrophages (alveolar macrophages in the lungs, Kupffer cells in the liver, dendritic and stellate cells).
The process occurs in stages: first, the microbe adheres to the phagocyte (adhesion), then it is engulfed, after which a potent microbicidal system is activated inside the cell. It is divided into oxygen-dependent and oxygen-independent subsystems, which ultimately inactivate and destroy the pathogen.