What Is the Immune System and How Does It Work? The Body’s Defense Mechanism

Countless microorganisms surround us. Some provide benefits, while others disrupt normal body functioning and cause harm. We call beneficial microorganisms the microbiome . Disease-causing microorganisms are called pathogens . The body has defense mechanisms protecting it from harmful microorganisms or destroying those that enter. Special structures on pathogen surfaces allowing their recognition are called antigens . The defense system has two basic tasks: protect against pathogenic microorganisms and destroy dead, damaged or abnormal cells. These systems are generally called the immune system . The science studying them is immunology.

The immune system has two divisions: innate and acquired:

The innate immune systemis the initial defense against pathogens. It produces an immediate response common to all pathogens.

This defense has four main components;

1. physical and chemical barriers,
2. phagocytes,
3. inflammatory response
4. certain plasma proteins.

Physical and chemical barriersinclude lysozyme, a bacteria-killing enzyme in saliva, sweat and tears; highly acidic gastric juice; and the mucus layer trapping microorganisms.

Another line of defense, phagocytes, engulf and break down invading pathogens, presenting their antigens on their own surfaces to activate other immune cells. Phagocytes comprise three cell types: macrophages, neutrophils and eosinophils.

The inflammatory response occurs in injured regions vulnerable to microbial infection. It dilates local blood vessels, allowing plasma to leak and cause swelling. White blood cells also reach the infection site, producing inflammation.

Finally, certain plasma proteins destroy microorganisms. An example is the membrane attack complex (MAC), which makes holes in bacterial cell walls and breaks bacteria apart.

Unlike innate immunity, the acquired/adaptive immune system is specific to the microorganism. It produces a longer-lasting response and a faster, more effective defense on encountering the same microorganism again. This defense is based on proteins called antibodies . Antibodies are produced by one of the cell types responsible for acquired immunity, B cells . After encountering a pathogen, B cells become pathogen-specific plasma cells and memory cells . B cells and antibodies are responsible for humoral/fluid-based immunity. T cells are responsible for cellular immunity . For example, through MHC receptors they recognize virus-infected cells and enable their destruction. B and T cells must be tested during formation and modified not to attack normal body cells. If this testing mechanism malfunctions, immune cells attack the body’s cells and autoimmune diseases occur, such as multiple sclerosis (MS). A situation that might be called the opposite is cancer; one of the 10 hallmarks of canceris its ability to be invisible to the immune system. Because cancer cells arise from our own cells, B and T cells often fail to see them as sufficiently foreign. Cancer cells carry surface molecules such as PD-1 and PD-L1 to appear friendly. New-generation cancer immunotherapiesare antibody drugs blocking these false hands of friendship.

In the figure below, cellular immunity is illustrated.

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In the figure below, the development of immune cells from blood stem cells is illustrated.