This article is from
Creation 43(3):20–24, July 2021

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Your ingenious immune system

Cleverly configured to devour, deactivate, and destroy

by Philip Bell

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Imagine a nation-state which regularly faces serious, even destructive, threats. In jeopardy from external forces and internal sabotage, it operates under strict martial law, with capital punishment for foreign invaders or rogue citizens. There are specialist centres for essential education and training, as well as detention centres, waste disposal facilities, and efficient transport systems.

The state ensures its own survival by ruthlessly policing its borders and neutralizing any menace. This requires surveillance and security checks. Its sophisticated identification system lets it detect and eliminate terrorists, while protecting innocent citizens. Individuals with particular skills, some at elite level, are its police, soldiers, and special operatives. High-level communication ensures coordinated responses, and pre-emptive strategies are in place to efficiently counter dangerous incursions.

All this has parallels with your body’s ingenious immune system. It resolves faults arising in your own cells or tissues which would otherwise compromise your safety. It protects you against external environmental perils. And your immune system deals decisively with anything that breaches your outer defences, preventing it from causing havoc inside the body.

Especially hazardous are tiny pathogenic (disease-causing) organisms. All were created harmless originally, and most types are benign to this day. But since the Curse (Genesis 3), innumerable invaders relentlessly assault humans: a vast variety of bacteria, viruses, fungi, single-celled protozoa, and multicellular parasites. Without an immune system, burns and scalds could easily be life-threatening, while small cuts would definitely prove fatal. Adam would have had an immune system pre-Fall, performing useful functions already.¹

There are two major types of defence or immunity in your body. The first occurs rapidly and is innate: the mechanisms you are born with. These are vital, but are not aimed at individual threats, such as a specific disease. The second takes longer to get up to speed but is adaptive. That is, it responds individually to specific invaders, such as a particular disease organism, by virtue of a highly specific threat-recognition and memory-retrieval system. What follows is a greatly simplified description of your immune system, but sufficient to help us grasp the ‘big picture’.

1. Innate immunity

Your innate (‘built-in’) immunity does not change with repeated attacks or exposure to threats. It takes various forms—mechanical, chemical, and cellular defences—each genetically programmed to immediately deal with a host of potential nasties (see fig. 1).

Smart skin and secretions

Unbroken skin is the first line of defence, an impermeable barrier between your body and the outside world, like a border control against foreign invaders. However, we are vulnerable at our unavoidable skin openings. These are our eyes, nose, mouth, anus, genital openings, and urethra (for passing urine).² Your airways are at risk from dust particles and microfibres (e.g., coal, hay, silica, plastics, asbestos, and textiles). Fortunately, all the body’s orifices are lined with moist mucous membranes to trap such particles as well as invading microorganisms.

Your skin and moist membranes produce various chemical secretions to immobilize or kill pathogenic organisms. The lining of the major airway branches in your lungs has mucus to trap alien debris, and hair-like cilia to sweep these out towards the throat, sometimes aided by coughing.³

The oily sebum produced by your skin’s sebaceous glands contains lactic acid and fatty acids, which inhibit bacterial growth.⁴ Lysozyme is an enzyme especially found in your tears. It can inactivate certain bacteria to help prevent eye infections. Your strong stomach acid, important for digestion, also destroys pathogens that would otherwise harm you. Urinating flushes the lining of your urinary tract clean. And there is much more besides.

Savvy cells with skills

Many specialized cells serve important roles in your immune system. These are your ‘white blood cells’ (leucocytes), constituting 1% of blood volume (see fig. 2). After developing in the bone marrow, they deploy in your blood, lymph (the colourless fluid in your lymphatic system), and various tissues and organs; e.g., spleen, thymus, and lymph nodes (see fig. 1). Those called lymphocytes are central to your adaptive immune response (discussed shortly). The types and roles of leucocytes are truly bewildering!⁵

 

Those especially relevant to innate immunity are phagocytes (‘cells that eat’). There are several important types; all can engulf and destroy foreign material/invaders:

Granulocytes are of three main types:

Neutrophils (up to 70% of leucocytes) are like privates in an army, your first line of defence against marauding microbes. They engulf and digest any foreign body.

Eosinophils (2–3%) are ‘higher-ranking, specialist soldiers’. They dampen inflammation, are involved in allergic reactions, and are especially active against parasites.

Basophils (<1%) are also specialists, active against bacteria and parasites. They produce histamine, releasing it if an allergen (e.g., pollen) enters your body. It makes your capillaries ‘leaky’ so neutrophils are more readily recruited to infected areas—also causing watering of your eyes and a runny nose!

Immunity is complex, and there is overlap in the functions of these three granulocyte types.

Macrophages (‘big eaters’) are your body’s main scavenger cells, like waste-disposal workers. Found throughout the body, they have different names; e.g., Langerhans cells (in skin), and Kupffer cells (liver).

Monocytes (2–8% of leucocytes) are equipped to recognize perils, and may change into other cell types (e.g., macrophages), when local chemical signals dictate. Like basophils, monocytes initiate inflammation, but also resolve it when it has served its purpose—they mop up dead or infected tissue.

Some of these ‘cell eaters’ may go on to process a foreign antigen and present it to lymphocytes (see ‘Awesome antibodies’); in this respect, their role overlaps with adaptive immunity.

The actions of phagocytes are triggered by cell-signalling proteins called cytokines—like chemical messengers. Once released from sites of inflammation, injury, and infection, these circulate throughout your blood. Some cytokines prompt phagocytes (e.g., neutrophils) to squeeze through the tiny blood vessels (capillaries) near the danger zone. Once in the vicinity, they do a clean-up job by ‘gorging themselves’ on broken cells and foreign material.

NK (natural killer) cells are also part of your innate immunity. They are special lymphocytes which know to avoid normal healthy cells, but attack rogue cancer cells or cells infected with viruses. They execute their targets without the need for antibodies (see ‘Awesome antibodies’—hereafter ‘AA’).

2. Adaptive (acquired or specific) immunity

Unlike innate immunity, adaptive immunity is specific to a particular type of invader, so it’s ultimately more powerful. However, it’s slower to respond, because the cellular agents must be briefed, then deployed, before carrying out their mission.

Infections may seriously jeopardize health; e.g., the measles virus or diphtheria bacterium. Sometimes, your innate immunity simply cannot keep such enemies at bay; your adaptive immunity makes the difference between recovery and death (or long-term health damage). You encounter such hazards repeatedly during your lifetime.

Wonderfully, there is a sophisticated mechanism for ridding your body of disease. It also ‘commits to memory’ important details of the culprit organism, so learning from the first encounter in case of future assaults. All this is made possible by pre-programmed lymphocytes, and weapons called antibodies (see AA and fig. 3a and 3b).

B-cells and T-cells

Lymphocytes, the operatives in adaptive immunity, include B-cells (B-lymphocytes) and T-cells—like police trained in various specialist areas of crimefighting. Some are shortlived, others live for months or years, accounting for the ability to ‘remember’ invaders long-term.

T-cells are so-called because they are ‘educated’ to maturity in your thymus (a gland overlying the heart). Some have an auxiliary function (T-helper cells) and some become assassins (T-cytotoxic cells).

B-cells do not get their name from the bone marrow (as one might think) but from the bursa of Fabricius, the organ in birds where their development occurs (and where they were first discovered). They are activated by each having molecules of its specific antibody embedded in its surface membrane (with the so-called ‘Fab’ arms sticking out; see fig. 3a). And each antibody is associated with a special membrane protein (CD79) to make a B-cell receptor.⁶ This happens in the spleen and lymph nodes. The great majority of circulating B-cells never see any action. However, once a B-cell receptor finds its match, it’s all systems go.

The fight is on!

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You are under constant assault from billions of foreign antigens (see AA). The day arrives: a real enemy incursion by some type of disease-causing agent. An alien antigen has been identified and the B-cell responsible immediately begins dividing. This happens over and over, producing a colossal proliferation of armed-and-dangerous agents called plasma cells. Some of the T-helper cells assist the B-cells in this process of ‘clonal expansion’. Now, however, although the plasma cells produce exactly the same antibodies, they don’t retain them as membrane receptors. Instead, the antibodies are released into the blood in their millions. The invaders’ days are numbered. The antibodies are now on a search-and-destroy mission, which will not cease until every enemy is eliminated and the infection conquered.

After recovering from the infection, you still have those plasma cells, which act as ‘memory cells’. With respect to that specific infectious agent, your body is in a state of higher alert than before. If that agent should rear its ugly head again, your memory cells will immediately recognize the danger: they will rapidly proliferate, so releasing masses of those special antibodies specific to that one type of invader. I.e., you are ‘immune’ to that disease. In our nation-state metaphor, repeated invasion attempts by a known aggressor cause nationwide mobilization of ruthless, deadly special agents. This is how vaccines work; by harmlessly mimicking an invasion by a particular pathogen, they pre-prime your immune system for when the real thing happens. In this fallen world, they protect us against some of the world’s most dangerous diseases. They have effectively eradicated some diseases in all or much of the world.⁷

So much more is going on

We’ve really only scratched the surface. Both innate and adaptive immunity are much more complex than shown here. Your leucocytes secrete many types of cytokines (see above). These fall into several main categories: CSFs, TNFs, interleukins, and interferons. All have complex roles in immunity. For instance, T-cells produce various interferons (IFN), crucial in antiviral resistance. One sort, IFNγ, activates and recruits macrophages and NK cells to fight viruses.

Since some invading cells are too big to be devoured by phagocytes, T-cytotoxic and NK cells punch holes in their membranes (using a ‘weapon’ called perforin). The target cells autodestruct,⁸ after which macrophages engulf any remaining debris.

Also, so that all the cells, antibodies, and cytokines get to where they’re needed, your body has a transport network par excellence. Your circulatory system ensures that your dedicated immune system cells (‘police’ and ‘crack troops’) rapidly reach sites of disturbance, damage, or infection. Space prevents discussion of the lymph nodes, ducts, and vessels of the lymphatic system, but think of the nodes as detention centres where insurgents are detained and eliminated.

In summary, whatever the menace, dedicated cells are always on hand to devour, deactivate, and destroy it. Your ingenious immune system is truly a design marvel.

 

Kudos to the Creator

The immune system is astonishingly sophisticated. It speaks eloquently of the super-intelligent mind of its Designer. Our awesome Creator God justly deserves our worship. However, there is one infection against which our ingenious immune system is entirely useless: the fatal disease of sin (Rom. 3:23; 6:23). Your only hope of overcoming that threat (Heb. 9:27) is to receive the complete forgiveness and pardon of your Creator, through genuine repentance and faith in Jesus Christ (Rom. 10:9).