Adog and a pig attempt to mate. we are aware that these are two different animal species. but what is the best explanation for the barrier that makes it impossible for them to successfully mate?

Dog and pig belongs to different species. The male reproductive cell or male gamete or sperm cannot fertilize with the female reproductive cell or female gamete or ova due to prezygotic barrier. The barrier could prevent fertilization, so that a zygote cannot be formed to produce hybrid.

the electromagnetic spectrum includes, from longest wavelength to shortest: radio waves, microwaves, infrared, optical, ultraviolet, x-rays, and gamma-rays.

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.