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The Defense mechanisms against infections

Nobody can survive for long without immune system. This line of defense protects us not only from unwelcome hosts like viruses, bacteria, and parasites but also from our own cells, which have changed due to diseases such as cancer. Due to the seasonal wave of flu, the immune system has often been called into question and even a little unfairly disparaged. Here are some surprising facts, often little known, which will make you appreciate it more.

It is distributed throughout the body

The dense surveillance network of the immune system includes a series of organs responsible for the production of bi cells such as spleen, bone marrow, lymph nodes, tonsils, thymus, tissues, and circulating cells. They communicate with the help of the lymphatic vessels. The immune cells are also distributed in all the tissues of the body, which they reach due to the blood circulation. With such a massive deployment of forces, it is difficult for a pathogen to go unnoticed.

Its flagship weapons are in the blood

In the arsenal of cells that guard the body, phagocytes and lymphocytes are mainly distinguished. The former, which develop in the bone marrow, constitute a first non-specific defensive line (not specified) and incorporate molecules into their cytoplasm, which, left free to circulate, could create problems. Similar cells with the same functions are also found in very elementary organisms. Pathogens that manage to overcome this first barrier encounter a specialized defense. Lymphocytes are capable of generating and modifying antibodies that recognize specific antigens on the surface of pathogens, and of neutralizing them. Only invertebrates did this second level of "tailor-made" defense develop.

Host defenses that protect against infections include

- Natural barriers (e.g., skin, mucous membranes)

- Non-specific immune responses (e.g., phagocytes, neutrophils, macrophages)

- Specific immune responses (e.g., antibodies, lymphocytes)

Natural barriers


The skin generally blocks invasion by microorganisms unless it is physically damaged (e.g., by arthropod vectors, lesions, IV catheters, surgical incisions). The exceptions include the following:

- Human papillomavirus, which can infect intact skin, causing warts

- Some parasites (e.g., Schistosoma mansoni, Strongyloides, and stercoralis 


Many mucous membranes are coated with secretions with antimicrobial properties. These secretions, such as cervical mucus, prostate fluid, and tears, contain lysozyme, which cleaves the bond of muramic acid on the walls of bacterial cells, especially Gram-positive ones. The secretions also contain immunoglobulins, mainly secretory IgG and IgA, which prevent the adhesion of microorganisms to host cells and proteins that bind to iron, which is essential for many microorganisms.

Respiratory tract

The respiratory system is equipped with filters in the upper respiratory tract. If invading microorganisms reach the tracheobronchial tree, the mucociliary epithelium pushes those outwards away from the lung. Coughing is also a useful mechanism for removing microorganisms. When microorganisms reach the alveoli, they are engulfed by alveolar macrophages and tissue histiocytes. However, these defenses can be overcome due to the high number of microorganisms that reach the alveolus. That may be due to the presence of air pollutants (e.g., smoke from a cigarette) for factors that interfere with defense mechanisms (e.g., endotracheal intubation or tracheotomy), cystic fibrosis).

Gastrointestinal tract

The barriers of the gastrointestinal tract are represented by the acidic pH of the stomach and by the antibacterial activity of the pancreatic enzymes, bile, and intestinal secretions. Peristalsis and physiological desquamation of epithelial cells contribute to the removal of microorganisms. When peristalsis has slowed down (e.g., in those who have used substances such as belladonna or opiate alkaloids), the removal of microorganisms is delayed. Impaired gastrointestinal defense mechanisms may predispose patients to particular infections such as gastric achlorhydria predisposes to salmonellosis. Normal intestinal flora can inhibit pathogens; alterations of the flora following the intake of antibiotics can allow an excessive proliferation of pathogenic microorganisms, such as (e.g., Salmonella Typhimurium).

Genitourinary tract

The barriers of the genitourinary tract are represented by the length of the urethra (20 cm) in men. The kidney also produces large quantities of Tamm-Horsfall mucoprotein, which by binding to certain bacteria, facilitates their removal.

Nonspecific immune responses (innate immune responses)

The cytokines (including IL-1, IL-6, TNF-alpha, interferon-gamma) produced principally by macrophages, and activated lymphocytes mediate an acute phase response that develops regardless of the causative microorganism. This response includes fever and increased bone marrow production of neutrophils. Endothelial cells also produce large quantities of IL-8, which recalls neutrophils.

The inflammatory response directs the components of the immune system towards the sites of damage or infection and manifests its action with an increase in blood supply and vascular permeability, allowing chemotactic peptides, neutrophils, and mononuclear leukocytes to leave the intravascular compartment.

Phagocytes (e.g., neutrophils and macrophages) limit microbial spread by engulfing microorganisms. These move by chemotaxis towards the microorganisms and incorporate them, by releasing the phagocytic lysosomal content that serves to destroy them. Phagocytes generate oxidation products, such as hydrogen peroxide, which kill ingested microorganisms. In the presence of quantitative or qualitative defects in neutrophils, an infection may be characterized by longer duration, a relapsing pattern, and a slower response to antibiotics. The main responsible pathogens are staphylococci, Gram-negative bacteria, and fungi.

Specific immune responses (Adaptive immune responses)

After infection, the host can produce various antibodies (i.e., complex glycoproteins known as immunoglobulins) that bind to specific microbial antigenic targets. It can help eliminate the infecting microorganism by recalling the host's GB and activating the complement system.

The complement system destroys the wall of the bacterial cell of infecting microorganisms, generally through the classical route. The complement can also be activated on the surface of some microorganisms through the alternative route. It can also promote the deposition of substances called opsonins (e.g., the complement protein C3b), on the surface of microorganisms that promote phagocytosis. Opsonization is an important mechanism for the eradication of capsule microorganisms, such as pneumococci and meningococci.

Genetic factors of the host

For many pathogens, the host's genetic makeup influences the host's susceptibility and the resulting morbidity and mortality. For exampleimmune system, patients who have deficiencies in complement terminal components (C5-C8, and perhaps C9) have a higher susceptibility to infections. 

Author: Vicki Lezama

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