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The body's vital defense force
Compare Animal Blood Cells
Watch or click through the pictures to compare blood cells. How do the different animal cells look similar? What differences do you notice?
Red blood cells carry out the same task in all animals: ferrying oxygen to other cells in the body. But the shape and size of RBCs differs from animal to animal. Some have round-looking RBCs, others have more oval-shaped cells. Some animals' RBCs lose their nuclei, others don't.

This slideshow contains pictures of blood cells from several different animals, all photographed at the same magnification. The different colors in the pictures are caused by the light and conditions used for the picture.

On a microscope slide, each animal's blood looks like human blood, a crimson red. What you can't see-until you put that slide under the microscope-is the shape and size of the cells.

What is immunity?
The protection from bacterial and viral infections conferred by your white cells is called immunity. Your immune system has two primary methods of protecting you, innate and acquired immunity, and they involve different kinds of white blood cells.

Most of us are born with a set of cells already prepared to fight off viruses and bacteria, giving us a built-in system of innate immunity. These cells include the granulocytes and the macrophages, both of which stop potential invaders by engulfing and digesting them. Cells like these are “preprogrammed” to recognize a huge number of pathogens, and are able to discern invaders from cells that belong to the body. Both granulocytes and macrophages rush to the site of infection or injury, ready to defend the home front. The swelling you find around a wound (or, when you’re sick, in your lymph nodes) is this army of white blood cells on the scene.

Granulocytes get their name from tiny granules within them that hold the potent chemicals used to destroy the microorganisms they’ve consumed. Like RBCs, granulocytes are produced in bone marrow—but they have a much shorter lifespan, usually around 6 hours. Granulocytes can eat one or several bacteria, but then die after releasing their toxic granules.

Macrophages, in contrast, can live for decades. These are large, malleable cells that can stretch themselves out within tissues as they look for marauders, and can eat many microorganisms. Not only do they kill the bugs, they also recycle their components (such as DNA and proteins), and parts of dead cells (like spent granulocytes).

The innate immune system protects us from many potential pathogens, but babies and children still get sick. In fact, they get sick more often than middle-aged adults. That’s because we also gain acquired or adaptive immunity as we get older. Some white cells, called T cells, have a kind of cellular memory for invaders they’ve encountered before. They can help spot a previously troublesome microorganism and jump-start an immune response to it.

There are several kinds of T cells: some signal that an invader is present, others can sidle up to a foreign microorganism and kill it. Still others regulate the immune response to keep it from getting out of control. Vaccines work by invoking acquired immunity: a person receives a small amount of a virus that has been weakened in the laboratory. As they fight the weakened virus, some of the T cells “learn” the characteristics of the invader and so can fend it off, should it invade again. In many cases, these “memory” T cells remain with a person their entire life. A person who’s had mumps or chicken pox, for example, develops a strong acquired immunity, and is protected from getting the disease again.

Next: When the immune system goes awry »