|Scanning electron micrograph of blood|
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.