The Immune System and Body Defenses

Barriers: the first line of defense

A pathogen is an organism or virus that causes disease: bacteria, viruses, fungi and some parasites. The best way to fight a pathogen is to keep it out. Your body has barriers that work the same way against every pathogen, so we call them part of the innate immune response (innate means "you are born with it").

• Skin: a tough physical barrier. Unbroken skin keeps most pathogens out.

• Mucus and cilia: mucus in your nose and airways traps germs. Tiny hair-like cilia sweep it out so you can cough, sneeze or swallow it.

• Stomach acid: a very acidic fluid that kills many pathogens in food and drink.

• Tears and saliva contain enzymes that break down bacteria.

What these have in common: they are nonspecific. They block or destroy any invader, and they do not need to "recognize" a particular germ.

Inflammation and white blood cells

When a pathogen gets past the barriers, the innate response keeps going. Think of Emma's scraped knee: within an hour it is red, warm and a little swollen. That is inflammation.

1. Damaged cells and invaders release chemical signals.

2. Blood vessels nearby widen and become leakier. More blood flows in, which causes redness and heat.

3. Fluid and white blood cells leak into the tissue, which causes swelling.

4. White blood cells called phagocytes (such as neutrophils and macrophages) engulf and digest pathogens (phagocytosis). The cells also clear away dead tissue.

Inflammation helps because it brings defenders to the exact spot of the injury and starts the repair. Heat and swelling are signs the defense is working, not that something has gone wrong.

Adaptive immunity: B cells, antibodies and memory

Some pathogens survive the innate response. Then the adaptive immune response begins. It is slower, but it is specific: it targets one particular pathogen, and it creates memory.

Every pathogen carries molecules called antigens. Your immune system uses them to recognize the pathogen.

• B cells (a type of white blood cell) are activated by a matching antigen. They multiply and become plasma cells that make antibodies.

• An antibody is a protein that binds to one specific antigen, like a key fitting a lock. It marks the pathogen for destruction, clumps pathogens together, or blocks a virus from entering cells.

• T cells (another white blood cell) help direct the response and destroy infected body cells.

• Some B and T cells become memory cells. They stay in your body for years.

Innate versus adaptive, and primary versus secondary response

Why is adaptive slower? On a first exposure, very few B or T cells match the antigen. They must be found, activated and multiplied before they can make enough antibodies.

Memory is the big payoff. Look at antibody levels after two exposures to the same pathogen:

• First exposure (primary response): slow start, a modest peak, then a decline.

• Second exposure (secondary response): memory cells already exist, so antibodies rise within days to a much higher level.

Viruses, spread of disease and vaccines

A virus is not a cell. It cannot make proteins or copy itself alone, so it must enter a host cell and use that cell's machinery to reproduce. That is why viruses make us sick: infected cells are damaged or destroyed.

Pathogens spread in several ways:

• Droplets and aerosols: a cough or sneeze releases tiny drops that others breathe in.

• Contact with surfaces: you touch a doorknob with virus on it, then touch your eyes, nose or mouth.

• Direct contact, contaminated food or water, and bites from animals such as mosquitoes.

Covering coughs, washing hands and staying home when sick break these chains.

A vaccine gives your immune system practice without the disease. It contains a weakened or inactivated virus, or a piece of one (an antigen). Your B and T cells respond and make memory cells. Later, if you meet the real virus, memory cells respond quickly, so you are often not sick at all or only mildly so. The vaccine does not kill the virus. It trains your immune system.

Some viruses, like influenza, mutate quickly. Their antigens change, so old memory cells and antibodies may no longer match. That is why the flu vaccine is updated each year.

Body systems working together

Defense is a team effort. Say Mateo steps on a rusty nail:

• Integumentary system (skin): the barrier is broken, which lets bacteria in.

• Nervous system: pain receptors send signals to the brain so he moves away and cares for the wound.

• Circulatory system: blood vessels widen and carry white blood cells, platelets and clotting factors to the wound. Clots stop bleeding and seal it.

• Immune system: phagocytes engulf bacteria (innate). B and T cells then make antibodies and memory cells (adaptive).

• Lymphatic system: lymph carries pathogens to lymph nodes, where B and T cells meet them and are activated.

Innate steps (inflammation and phagocytes) happen in minutes to hours. Clotting also happens quickly, but it is part of the circulatory system's response, not the immune response. The adaptive response needs days to find and multiply the right cells. A deep puncture may also carry tetanus bacteria, so a vaccine booster is often given.

Summary

• Innate defenses (skin, mucus, stomach acid, inflammation, phagocytes) act fast and work against all pathogens.

• Inflammation causes redness, heat and swelling, and brings white blood cells to an injury.

• Adaptive defenses are slower but specific. B cells make antibodies, proteins that bind one antigen. T cells help and attack infected cells.

• Memory cells make the second response faster and much stronger.

• Vaccines expose you to an antigen, not the disease, so you form memory cells. Fast-mutating viruses need updated vaccines.

• Many body systems work together to detect injury, limit infection and repair.

Back to the start: Emma's knee healed because skin and inflammation handled the bacteria right away, and her adaptive response cleared anything left.