Before understanding innate immunity, let us first understand the meaning of immunity. Immunity is the ability of the body to resist and defend itself against infections and diseases. Susceptibility, on the other hand, means that an individual is more likely to develop an infection because the body’s resistance is insufficient.
1. Introduction to Innate Immunity
Our body is continuously exposed to microorganisms such as bacteria, viruses, fungi and parasites. However, not every exposure results in an infection. This is because our body possesses several defence mechanisms that prevent microorganisms from entering, multiplying and causing disease.
The first major component of this protection is innate immunity.
Innate immunity is the body’s natural defence mechanism that is present from birth. It responds rapidly to infection and provides protection against a broad range of pathogens.
Unlike adaptive immunity, which recognizes specific antigens and develops immunological memory, innate immunity relies mainly on recognizing common features of microbes and signs of tissue damage.
For our discussion, we will focus on the barriers, cells and protective molecules involved in innate immunity.
2. The First and Second Lines of Defence
The body’s innate defence can be understood through two major levels.
The first line of defence consists of barriers that prevent pathogens from entering the body. These include the skin, mucous membranes, chemical secretions and normal microbiota.
The second line of defence becomes important when pathogens cross these barriers. It includes immune cells, inflammation, phagocytosis, fever and antimicrobial proteins such as complement and interferons.
Think of the body as a protected building. The skin and mucous membranes act like walls and gates. If a pathogen crosses these barriers, internal defence mechanisms become active to locate and eliminate it.
Let us examine these mechanisms one by one.
3. First Line of Defence: Barriers
The first line of defence includes physical, chemical and biological barriers.
A. Physical barriers
The skin is one of the most important protective barriers of the body. Its outer layer contains tightly packed cells and keratin, which help prevent the entry of most microorganisms.
The continuous shedding of surface skin cells also helps remove microbes attached to the skin. In addition, the relative dryness of the skin makes it difficult for many microorganisms to grow.
Mucous membranes line the respiratory, digestive and genitourinary tracts. These membranes produce mucus, which traps microbes and particles and prevents them from reaching deeper tissues.
In the respiratory tract, tiny hair-like structures called cilia move mucus towards the throat, helping remove trapped microorganisms and particles. Coughing and sneezing provide additional protection by expelling unwanted material.
Other protective mechanisms include the flushing action of tears and urine, the movement of intestinal contents and the removal of waste through defecation.
Together, these physical mechanisms reduce the chances of pathogens entering and establishing an infection.
B. Chemical barriers
Our body also produces chemicals that inhibit or destroy microorganisms.
Sebum, an oily secretion produced by sebaceous glands, helps maintain the skin’s protective surface and creates conditions that inhibit the growth of some microbes.
Lysozyme is an antimicrobial enzyme present in tears, saliva and other secretions. It damages bacterial cell walls by breaking bonds in peptidoglycan.
The stomach produces gastric acid, which destroys or inhibits many microorganisms entering with food.
Other secretions and antimicrobial substances present on mucosal surfaces also help prevent microbial growth.
These chemical barriers provide protection without requiring a highly specific response against an individual pathogen.
C. Biological barriers: Normal microbiota
Our skin, intestine and other body surfaces contain communities of microorganisms known as the normal microbiota.
Many of these microorganisms are beneficial. They compete with pathogens for nutrients and attachment sites, and some produce substances that inhibit competing microbes.
For example, normal intestinal microbiota helps limit the establishment of certain disease-causing microorganisms.
Therefore, the first line of defence is not limited to physical structures. It also includes chemical conditions and beneficial microorganisms that make it difficult for pathogens to establish themselves.
Key point: The first line of defence mainly prevents pathogens from entering the body’s internal tissues.
4. Recognition of Pathogens: PAMPs and PRRs
What happens when a pathogen crosses the skin or mucous membranes?
The body must recognize that something potentially harmful has entered.
Innate immune cells possess receptors that recognize common molecular features of microorganisms. These receptors are called pattern recognition receptors, or PRRs.
The microbial structures recognized by these receptors are called pathogen-associated molecular patterns, or PAMPs.
Examples include lipopolysaccharide, or LPS, found in the outer membrane of many Gram-negative bacteria, and certain forms of microbial nucleic acids.
One important family of PRRs is the Toll-like receptors, or TLRs. Different TLRs recognize different microbial components. Some are present on the cell surface, while others are located inside cellular compartments.
When these receptors recognize microbial components, they activate signals inside the cell. These signals help initiate inflammation and stimulate the production of protective molecules.
Innate immunity can also respond to molecules released by damaged or stressed cells. These are called damage-associated molecular patterns, or DAMPs.
Thus, innate immunity can respond to both microbial invasion and tissue damage.
Imagine a security system that recognizes common signs of intrusion rather than identifying every intruder individually. PRRs perform a similar role by detecting patterns associated with infection or damage.
5. Cells of Innate Immunity
Several types of immune cells cooperate to defend the body. Each has a different function.
Neutrophils
Neutrophils are important early responders during many bacterial infections. They move rapidly from the blood into infected tissues, engulf microorganisms and destroy them.
They are particularly important in acute inflammation.
Monocytes and macrophages
Monocytes circulate in the blood and can enter tissues, where they may differentiate into macrophages.
Macrophages engulf microorganisms, remove dead cells and damaged tissue, and release signalling molecules that help coordinate inflammation.
Some macrophages reside in particular tissues, while others are recruited from circulating monocytes during inflammation.
Dendritic cells
Dendritic cells detect and capture microbial material. They also help connect innate immunity with adaptive immunity by presenting antigen-derived information to T cells.
For this lecture, remember that their immediate role includes pathogen detection and coordination of the wider immune response.
Natural killer cells
Natural killer, or NK, cells are lymphocytes that belong to innate immunity. They help control virus-infected cells and certain tumour cells.
They can kill abnormal cells by releasing substances such as perforin and granzymes, which help induce the death of target cells.
Mast cells and basophils
Mast cells are present in many tissues, while basophils circulate in the blood. Both can release histamine and other mediators that contribute to inflammation.
They are also involved in allergic responses.
Eosinophils
Eosinophils are particularly important in defence against certain parasitic worms. They release granule contents that can damage parasites and also participate in allergic inflammation.
Together, these cells provide surveillance, pathogen elimination and communication during innate immune responses.
6. Phagocytosis: Engulfment and Destruction of Microbes
One of the most important mechanisms of innate immunity is phagocytosis.
Phagocytosis is the process by which certain cells engulf particles or microorganisms and digest them inside the cell.
Neutrophils and macrophages are major phagocytic cells.
The process can be explained in five steps.
Step 1: Chemotaxis
The phagocyte moves towards the site of infection by following chemical signals released by microbes, damaged tissues and other immune cells.
Step 2: Recognition and attachment
Receptors on the phagocyte recognize the microorganism and help the cell attach to it. Proteins such as complement fragments can coat microbes and make them easier to recognize and engulf. This is called opsonization.
Step 3: Engulfment
The phagocyte extends its cell membrane around the microorganism and encloses it inside a vesicle called a phagosome.
Step 4: Digestion and killing
The phagosome fuses with a lysosome, forming a phagolysosome. Lysosomal enzymes and other antimicrobial substances help destroy and digest the microorganism.
Phagocytes can also produce reactive oxygen species, which contribute to microbial killing.
Step 5: Removal of waste
The remaining indigestible material may be removed from the cell.
Phagocytosis is essential because it helps eliminate pathogens that have crossed the body’s external barriers.
However, some microbes have developed ways to escape this process. Certain bacteria have capsules that make engulfment more difficult, while others can interfere with the destruction of microbes inside phagocytes.
This illustrates the ongoing interaction between microbial survival strategies and host defence mechanisms.
7. Inflammation: Bringing Defence to the Site of Infection
Inflammation is a protective response to infection or tissue injury.
Its main purpose is to recruit immune cells and protective molecules to the affected area, eliminate harmful agents and initiate tissue repair.
The four classical signs of inflammation are:
- Redness — caused mainly by increased blood flow.
- Heat — also associated with increased blood flow.
- Swelling — caused by fluid accumulating in the tissues.
- Pain — caused by inflammatory mediators and pressure on sensory nerve endings.
Let us consider an example.
Suppose you accidentally cut your finger while preparing food. The cut damages the skin and may allow microorganisms to enter.
Cells in the injured tissue detect damage or infection and release chemical mediators. Mast cells can release histamine, while other cells produce cytokines and lipid mediators.
These substances cause blood vessels to dilate and become more permeable.
As a result, more blood reaches the affected area, and fluid and protective molecules can move from the blood into the tissues.
Immune cells, particularly neutrophils during many acute bacterial infections, are recruited to the site.
They leave the bloodstream by passing through the blood vessel wall. This movement is called diapedesis.
Once inside the tissue, these cells move towards the source of infection and help destroy microorganisms.
After the harmful agent is controlled, inflammatory activity should decline and tissue repair can proceed.
Inflammation is therefore beneficial when properly regulated. However, excessive or prolonged inflammation can damage healthy tissues.
8. Cytokines and Chemokines: Communication Between Immune Cells
Immune cells need to communicate with each other to coordinate an effective response.
They do this partly through chemical messengers called cytokines.
Cytokines are proteins that regulate immune-cell activity, inflammation and other body responses.
Some important examples are:
- TNF-α: Promotes inflammation and helps activate local defence responses.
- IL-1: Contributes to inflammation and fever.
- IL-6: Promotes inflammation and stimulates the production of acute-phase proteins by the liver.
- IL-12: Helps activate NK cells and supports certain cellular immune responses.
- Type I interferons: Help establish antiviral defences.
Chemokines are a group of signalling proteins that guide immune cells towards particular locations.
For example, CXCL8, historically called IL-8, attracts neutrophils to sites of inflammation.
Think of cytokines as messages that coordinate the response and chemokines as directional signals that help immune cells reach the correct location.
These messengers link pathogen recognition to inflammation, cell recruitment and other protective responses.
9. Complement System: Protective Proteins in the Blood
The complement system is a group of proteins found mainly in blood plasma and other body fluids. Many are produced by the liver.
These proteins circulate in inactive forms and become activated in a cascade, in which one activated component helps activate the next.
Complement can be activated through three main pathways:
- Alternative pathway: Can be activated on microbial surfaces.
- Lectin pathway: Begins when lectins, such as mannose-binding lectin, recognize particular sugars on microbial surfaces.
- Classical pathway: Commonly begins when antibodies are bound to their targets, linking complement to adaptive immunity.
For this lecture, the most important point is to understand the three major outcomes of complement activation.
A. Opsonization
The complement fragment C3b can attach to a microbial surface. Phagocytes recognize complement-coated microbes more efficiently and can engulf them more readily.
Thus, opsonization makes pathogens easier to phagocytose.
B. Inflammation and recruitment
Complement fragments such as C3a and C5a promote inflammatory responses. C5a is also an important signal for recruiting and activating immune cells.
These effects help bring additional defences to the site of infection.
C. Cytolysis
Some complement proteins assemble to form the membrane attack complex, or MAC.
The MAC creates a pore in the membrane of susceptible target microbes, which can lead to cell damage and lysis.
Therefore, complement helps the body through three major mechanisms: marking microbes for destruction, promoting inflammation and directly damaging susceptible microbes.
10. Interferons: Defence Against Viruses
Viruses multiply inside host cells, so the immune system must control infection while limiting its spread to neighbouring cells.
Interferons are important signalling proteins involved in this process.
When a cell detects a viral infection, it can produce type I interferons, mainly interferon-alpha and interferon-beta.
These interferons bind to receptors on the infected cell and neighbouring cells. The resulting signals stimulate the production of antiviral proteins.
These proteins can interfere with different stages of viral replication, helping cells establish an antiviral state.
This does not make neighbouring cells completely immune to infection. Rather, it makes them better prepared to resist viral multiplication.
Type III interferons also contribute to antiviral defence, particularly at epithelial barriers such as those of the respiratory and intestinal tracts.
Key point: Interferons help slow the spread of viral infection by warning cells and stimulating antiviral mechanisms.
11. Fever and the Acute-Phase Response
Fever is another response associated with innate immunity.
During infection, immune cells release cytokines such as IL-1 and TNF-α. These signals, together with other inflammatory mediators, act on regulatory mechanisms in the brain that control body temperature.
The temperature set point rises, producing fever.
A moderate fever can support host defence by affecting microbial growth and modifying immune activity. However, fever is not always beneficial, and very high or persistent fever can be harmful.
The body also changes the availability of nutrients needed by microorganisms.
For example, proteins such as lactoferrin bind iron, reducing the amount available to certain microbes. This is one example of nutritional immunity.
Another important response is the acute-phase response.
During inflammation, cytokines such as IL-6 stimulate the liver to produce acute-phase proteins. Examples include C-reactive protein (CRP), which can bind certain microbial or damaged-cell components, and mannose-binding lectin, which can contribute to complement activation.
The acute-phase response therefore connects inflammatory signalling, liver activity and antimicrobial defence.
12. How Pathogens Escape Innate Immunity
Although innate immunity is effective, microorganisms have evolved ways to avoid or resist its mechanisms.
Some bacteria possess capsules that reduce phagocytosis. Others interfere with the normal maturation of phagosomes or survive inside host cells.
Some pathogens can interfere with complement activation or prevent complement proteins from damaging them.
Viruses may also produce proteins that interfere with the host’s detection of viral infection or its interferon response.
These examples show that pathogens do not simply encounter passive barriers. They interact with the host’s defences and may develop mechanisms to survive.
When these defences are overwhelmed or become poorly regulated, severe illness can occur. For example, an uncontrolled systemic inflammatory response during infection can contribute to sepsis and organ dysfunction.
13. Summary: How the Components Work Together
Let us bring all the major components together.
First, physical, chemical and biological barriers help prevent pathogens from entering the body.
If a pathogen crosses these barriers, innate immune cells recognize microbial patterns through PRRs. Signs of tissue damage can also activate innate responses.
Recognition stimulates the release of cytokines and chemokines. These messengers coordinate inflammation and recruit immune cells.
Neutrophils and macrophages engulf and destroy microbes through phagocytosis. NK cells help eliminate certain infected or abnormal host cells.
Complement proteins enhance phagocytosis, promote inflammation and damage susceptible microbes. Interferons help neighbouring cells resist viral replication.
Fever and the acute-phase response provide additional systemic support. Once the infection is controlled, inflammation should resolve and tissue repair can proceed.
Finally, innate immunity also helps initiate adaptive immunity. Dendritic cells can present antigen-derived information to T cells, helping activate the more specific immune response that will be discussed in a later lecture.
To conclude, innate immunity is not a collection of independent mechanisms. It is a coordinated defence network in which barriers, recognition receptors, immune cells, chemical messengers and antimicrobial proteins work together to protect the body.