Role and functions of the complement system
The complement system plays a crucial role in the elimination of viruses and bacteria, as well as in the activation of phagocytic reactions. Its main functions include microbial opsonization, neutrophil chemotaxis, and antigen presentation.
In newborns, this mechanism has age-related features: complement activity is reduced by nearly half compared to adult values. This is due to low levels of C3 and C4 factors and membrane attack complex components (C8 and C9). Consequently, a weak local response is formed, increasing the risk of generalized infection. Nevertheless, complement levels reach normal values by the end of the first month of life.
Interferons and defense against viruses
A crucial component of humoral defense is interferons. These are specialized proteins that exhibit pronounced non-specific antiviral activity. They effectively combat viral agents, preventing their spread and replication in body tissues.
Features of the immune response in various infections
The nature of the body's defense directly depends on the type of pathogen:
- Bacterial exotoxins (tetanus, diphtheria, gas gangrene) are neutralized by specific antibodies—antitoxins. During primary invasion, this process is slow.
- Viruses during hematogenous spread are neutralized by humoral factors; when replicating at the site of entry, local immunity (IgA) is engaged; and during intracellular replication, the cellular response is critical.
- Fungal infections require the activation of predominantly cellular immunity.
- Protozoan infections are distinguished by a pronounced diversity of the pathogen's antigenic composition.
- Helminthic infestations are accompanied by the stimulation of IgE synthesis and the formation of a cellular infiltrate involving eosinophils, basophils, and mast cells.
Immunological memory
The formation of long-term defense is ensured by clones of long-lived lymphocytes, or memory cells. They are formed after primary contact with the antigen and allow the organism to develop immunity—ranging from short-term to lifelong.
Briefly: answers to topical questions
What other humoral factors of innate immunity exist, besides complement and interferons?
In addition to the complement system and interferons, humoral factors of innate immunity include antimicrobial peptides, cytokines, chemokines, and acute-phase proteins.
- Cytokines and chemokines are pro-inflammatory factors that regulate inflammation and the immune response.
- Antimicrobial peptides include lysozyme, defensins, and cathelicidins, which destroy the bacterial cell wall.
- Acute-phase reactants include C-reactive protein, proteases, protease inhibitors, blood clotting factors, and transport proteins.
They also include various enzymes secreted by macrophages, hormones, cationic proteins, proteoglycans, and arachidonic acid metabolites.
Through which pathways can the complement system be activated?
Activation of the complement system can occur via three main initiation pathways.
- Classical pathway is an antibody-dependent pathway initiated by the recognition of the antigen-antibody complex.
- Lectin pathway is triggered by the recognition of carbohydrates on the pathogen surface by mannose-binding lectin.
- Alternative pathway is initiated by the spontaneous hydrolysis of the C3 component.
The first phase of initiation concludes with the formation of C3/C5 convertases. Afterward, the mechanisms converge into a common terminal pathway, ensuring the second phase—target cell lysis—which is completely identical for all three activation pathways.
Class and exam questions
What are the main functions of the complement system?
The complement system provides microbial opsonization, neutrophil chemotaxis, antigen presentation, and participates in the activation of phagocytosis and the elimination of viruses and bacteria.
Why do newborns have an increased risk of generalized infections?
In newborns, complement activity is halved due to low levels of C3, C4 factors, and the membrane attack complex, leading to a weak local response.
How does the body defend against bacterial exotoxins?
Exotoxins are neutralized by specific antibodies—antitoxins, although during a primary infection, this process occurs slowly.
Which elements participate in the response to helminthic infestations?
In helminthic infestations, IgE synthesis is stimulated, and a cellular infiltrate is formed consisting of mononuclear phagocytes, lymphocytes, eosinophils, basophils, and mast cells.
What else to study on this topic
- Age-related dynamics of C3 and C4 factors in infants during their first year of life
- Mechanisms of action of the membrane attack complex
- The role of secretory IgA in localized viral infections
- Features of antigenic composition in protozoan infections
- Formation of memory lymphocyte clones in primary and secondary responses
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