In cases of immune system over activity, the body attacks and damages its own tissues (autoimmune diseases). Immune deficiency diseases decrease the body's ability to fight invaders, causing vulnerability to infections.
Standard deviation is a measure of spread (like Range, Variance, or IQR) so it tells you how wide or concentrated your data is. You can interpret it as how far away the typical/average observation is from the mean.
The average absolute deviation (or mean absolute deviation) of a data set is the average of the absolute deviations from a central point. It is a summary statistic of statistical dispersion or variability.