The human immune system has two divisions or arms that help fight off infection namely the innate or non - specific immunity and the acquired immune system also called specific immunity.
The non - specific immunity arm is the first line of defense against pathogens. It incorporates physical barriers which include the skin and mucous membranes. Pathogens usually cannot penetrate into the body unless the skin is broken. The mucus membranes trap foreign particles using the mucus they secrete and the intruders are expelled from the body.
The specific immunity arm is the second line of defense and attacks pathogens that have breached the first line of defense. It works by making memory cells, special lymphocytes that store memory of a pathogen when encountered for the first time. If the same type of pathogen invades the body a second time, the memory cells remember it and trigger a more rapid and vigorous response specific to that pathogen, neutralizing it through antibody production and destroying it.
Within the muscle cells, the space between the fibers is shortening and they begin to contract.