Infection and the Non-Specific Immune Response

When the body encounters a pathogen, its first line of defense is the non-specific, or innate, immune response.

Pathogens and Infections

A pathogen is a microbe such as a bacterium or virus that causes disease. Pathogens can invade healthy cells and tissues, leading to infection.
Pathogens use several methods to bypass the human body’s external defenses, including mechanical means (such as through wounds), biological means (like injecting through a mosquito bite), or transport via air or water.

Barrier Defenses

Barrier defenses, such as skin and mucus, act to block pathogen entry. The epithelial tissues lining the respiratory, digestive, and urogenital tracts are common entry points for pathogens and are protected by mucus.
Other physical and biochemical barriers like tears, saliva, and stomach acid also help to prevent pathogen entry and survival.

Innate Immune Response

Once a pathogen breaches these barriers, non-specific immune cells like white blood cells (leukocytes) are activated.
Phagocytes, such as neutrophils and macrophages, engulf and digest the pathogen in a process called phagocytosis.
Natural Killer (NK) cells can recognise and kill infected cells directly without needing to phagocytose the pathogen.
Certain types of leukocytes release interferons, proteins that interfere with viral reproduction inside host cells.

Inflammation and Fever

Invading pathogens trigger an inflammatory response, characterised by redness, heat, swelling, and pain, which helps to recruit more immune cells to the site of infection.
Inflammation is caused by chemical signals released by immune cells and damaged tissues, including histamine and cytokines. These signals increase blood flow and permeability of blood vessels.
Fever is a systemic response to infection, caused by cytokines acting on the temperature-control centre in the brain, that can enhance immune system function and hinder pathogen growth.

Complement System

The complement system consists of proteins in the blood that complement the activity of phagocytes, enhancing their ability to clear microbes and damaged cells, promote inflammation, and attack pathogen cell membranes.

Non-Specific Immune Limitations

While the non-specific immune defenses can handle a wide range of threats, they lack specificity and memory. Therefore, these mechanisms may not be able to fully neutralise a pathogen, especially one that the body has not encountered before.
The non-specific immune response is also not adaptive. If the body encounters the same pathogen again, the speed and strength of the non-specific immune response does not change.

Interaction with Specific Immunity

After the non-specific immune response has been triggered, dendritic cells present antigens (pieces of pathogen) to T cells of the adaptive immune system, launching a specific immune response. This sequence ensures a rapid first response to a pathogen, followed by a more tailored defense.

Remember, studying the interactions between pathogens and the immune system not only helps us understand disease processes but also guides the development of treatments and preventative measures like antibiotics, antivirals, and vaccines.