What is the immune systems first line of defense against invading pathogens

In many humans, exposing the skin to sunlight over prolonged periods of time results in the production of more pigment by the skin cells (tanning). this change in skin color provides evidence that -

What is the main fuel source for the work of the cell

The mixture of sperm and fluids from the seminal vesicles, prostate gland, and cowper's glands is called