Microbe-Host Interaction
The infectious process represents a specific form of symbiosis. Contact with a microbe does not always lead to disease. Normal microflora is vital for humans: it maintains optimal pH, helps organs perform their functions, and participates in metabolism. Destruction of beneficial bacteria (e.g., by antibiotics) leads to severe dysbiosis.
Pathology develops when the balance is disrupted and parasitism occurs—an antagonistic interaction causing harm to the host. Key properties of pathogens:
- Pathogenicity is the fundamental ability to cause disease.
- Virulence is the degree of pathogenicity of a specific strain.
- Invasiveness is the ability to multiply and spread through tissues.
- Toxicity is the production of damaging substances.
Natural Body Barriers
From a general biological perspective, microbes are part of our environment. To protect against their aggression, the body has built three lines of defense:
- Anatomical barriers: skin and mucous membranes. They physically block pathogens and are covered with secretions possessing antimicrobial properties (containing organic acids and the enzyme lysozyme).
- Physiological barriers: the acidic environment of gastric juice, intestinal peristalsis, continuous urine flow, coughing, and mucociliary clearance.
- Immune barriers: resident cells (macrophages and lymphocytes) patrolling the dermis, alveoli, and beneath mucous membranes, as well as the secretion of protective antibodies (IgA) on the epithelial surface.
Mechanisms of Damage and Immune Evasion
Upon invading tissues, pathogens cause inflammation and cell damage. Viruses integrate their genome into the host DNA, forcing the cell to synthesize new viral particles. Bacteria act differently: they attach to membranes using specific proteins (adhesins) and release toxins.
- Endotoxins are components of the cell wall of Gram-negative bacteria (lipopolysaccharides). They cause fever and a massive release of cytokines.
- Exotoxins are enzymes secreted outward by microbes (hemolysins, fibrinolysins, hyaluronidases).
To survive, microorganisms have developed defense mechanisms. They can hide inside cells, suppress the immune response, or form dense capsules. For example, the carbohydrate capsule of meningococci and pneumococci reliably protects their antigens from recognition and hinders phagocytosis.
Morphology: Primary Infection Complex
The site of pathogen entry is called the portal of entry. It is here that the initial reaction unfolds, forming the primary infection complex. This is an obligatory triad of changes for most infectious diseases:
- Primary affect is the focus of inflammation (often with necrosis) directly in the tissue to which the microbe is adapted.
- Lymphangitis is the inflammation of draining lymphatic vessels.
- Regional lymphadenitis is the inflammation of nearby lymph nodes where infection and toxins are transported via lymph flow.
It is important to understand that this complex is not merely a local problem, but a local manifestation of an already generalized infection. Simple surgical excision of the primary affect (e.g., hard chancre in syphilis) does not cure the patient.
The role of body reactivity
The outcome of encountering an infection depends on reactivity—the ability of the host organism to respond to a stimulus. Several response patterns are distinguished:
- Normergy is the standard reaction upon primary contact with a pathogen.
- Hyperergy is a violent allergic reaction (hypersensitivity). It helps localize the infection at the portal of entry and determines the general manifestations of the disease.
- Hypoergy is a reduced response intensity, where local specific changes come to the forefront.
- Anergy is the absence of a reaction. It can be protective (after successful vaccination) or pathological (in deep exhaustion of compensatory reserves).
Briefly: answers to topical questions
Which bacteria form a capsule to evade the immune response?
To evade the immune response, the carbohydrate capsule is formed by the following bacteria:
- Pneumococci (Streptococcus pneumoniae)
- Meningococci
- Haemophilus bacteria
The capsule protects bacterial antigens, hinders phagocytosis, and increases the virulence of microorganisms. Large parasites (e.g., tapeworm larvae) form cysts covered with a dense capsule, which also ensures morphological protection and inaccessibility to host immune reactions.
Which aggression enzymes are classified as bacterial exotoxins?
Bacterial exotoxins include various enzymes secreted by microorganisms:
- leukocidins;
- hemolysins;
- hyaluronidases;
- fibrinolysins.
Which infectious diseases are classified as natural focal diseases?
Natural focal infectious diseases include:
- Malaria — a biocenosis transmitted via an intermediate host, a mosquito of the genus Anopheles.
- Tularemia — a zoonotic natural focal disease.
- Plague — a zoonosis of wild animals.
- Tick-borne encephalitis — a zoonosis of wild animals.
- Rabies — a zoonosis of wild animals.
- Tick-borne borrelioses and arboviral infections — zoonoses of wild animals.
What morphological changes characterize the primary affect in tuberculosis?
The primary affect in tuberculosis is characterized by specific morphological changes depending on the route of infection.
- In aerogenic infection — a focus of caseous necrosis and surrounding serous inflammation forms in the lungs. The size varies from several alveoli to a segment. Localization is most often subpleural, causing concomitant fibrinous or serofibrinous pleuritis.
- In alimentary infection — the primary affect presents as an ulcer in the lymphoid tissue of the lower jejunum or cecum.
- In lesions of the skin or tonsils — the changes also manifest as an ulcer.
What physiological barriers protect the respiratory tract from infection?
The respiratory tract is protected against infection by the following physiological and reflex barriers:
- Mucociliary clearance — a transport mechanism in which the beating of epithelial cilia propels mucus with trapped particles upward.
- Cough reflex — ensures the evacuation of foreign bodies and mucus.
- Sneezing reflex — facilitates the mechanical removal of pathogens.
- Ventilation function — adequate pulmonary ventilation capacity acts as a self-cleaning factor.
These mechanisms ensure the elimination of microorganisms without the involvement of phagocytosis.
To remember the components of the primary infection complex, use the rule of three "A"s: Affect (focus) — Lymphangitis (pathway) — Lymphadenitis (outpost).
Class and exam questions
What are opportunistic infections?
These are diseases caused by the human resident opportunistic microflora during a sharp decrease in immunity (for example, in premature infants or after taking cytostatics). They lack strict specificity: the same microbe can affect different organs.
How does natural pathomorphosis differ from induced pathomorphosis?
Natural pathomorphosis is linked to microbial evolution (emergence of new viruses, growth in the proportion of opportunistic flora), whereas induced pathomorphosis is associated with socio-medical measures (vaccination, sanitation). Induced pathomorphosis is unstable: with a decline in living standards, "defeated" diseases return.
What is the natural focal nature of infections?
This is the confinement of certain diseases (biocenoses) to specific geographical regions. It is due to the fact that transmission of the pathogen strictly requires an intermediate host that lives only in that territory (e.g., the mosquito genus Anopheles in malaria).
What else to study on this topic
- Evolutionary and historical prerequisites for the formation of infections (waterborne, airborne droplet, zoonosis adaptation).
- The impact of socio-economic factors on the historical dynamics of morbidity.
- Classification of infectious diseases by source (anthroponoses, zoonoses, anthropozoonoses, biocenoses).
- Mechanisms of intracellular bacterial survival (disruption of protein synthesis, inhibition of oxidation).
- Clinical and morphological classification of infections by affected organ systems.
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