White blood cells - Immunity

White blood cells come in a number of varieties, each with specialized functions in the body. I mentioned before that they start out as lymphocytes - but really there are other specialized leukocytes (i.e. white blood cells) that aren't lymphocytes as well - specifically monocytes and the phils (neutrophils, eosinophils, basophils). Hematology (the study of the blood) is a field in its own right because of this complexity.

Scanning electron micrograph of blood

White blood cells are responsible for fighting infection and preventing invasion by foreign particles. They are constantly tasting the components in the blood to determine whether they belong to the body, and they do this through receptors on the cell surfaces. What happens if they find something that they don't recognize? An immune response.

Allergies are fast and severe immune responses to something the body doesn't recognize. This is why food intolerance can develop - slowly over time the immune cells become sensitive and then, bam, they're ready to attack and do. Allergies may also be due to the genetics of the proteins on the cell surfaces, which recognize certain food proteins as needing to react.

When the body lacks white blood cells, as in the case of HIV infection (the virus destroys the cells), immunodeficiency occurs. This makes the person susceptible to common infections. Normally we co-exist with bacteria, viruses, and fungi, like cytomegalovirus and strains of staph on our skin, but without the white blood cells to keep these pathogens in their place, they invade. Something as simple as the common cold can be deadly to a person with compromised immunity. And some people are born lacking the immunity they need - resulting in cases like that of the 'bubble boy' syndrome.

So now that we know the cell components of blood are important, what's the purpose of plasma? Read about that tomorrow.