Innate immunity . is found only in vertebrates is the first, and most general, mechanism of protection against pathogens depends on an infected animal's previous exposure to a pathogen is based on recognition of antigens that are specific to a pathogen

is the first, and most general, mechanism of protection against pathogens

Explanation:

Innate or natural immunity is one of the two types of immunity an organism can possess (the other being; acquired immunity). Innate immunity is the FIRST defensive response of an organism against pathogenic attacks. The acquired immune system is only activated when the innate immune system has been penetrated or surmounted.

For example, the skin, mucus secretion, inflammation etc. are all natural or innate defenses against foreign invaders. Therefore, according to this question, innate immunity the first, and most general i.e. commonly used, mechanism of protection against pathogens.

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!