Innate immunity
1. utilizes highly specific antigen receptors on b cells.
2. is based on recognition of antigens that are specific to different pathogens.
3. is activated immediately upon infection.
4. depends on a newly infected animal's previous exposure to the same pathogen.
5. is found only in vertebrate animals.

3. is activated immediately upon infection.

Explanation:

Innate immunity is the nonspecific immunity. These immune defenses are present at birth and do not involve specific recognition of a microbe. Being nonspecific in nature, it acts against all microbes.  Innate immunity includes the first and second lines of defense.

Some of the examples of components of innate immunity among the first line of defenses are physical and chemical barriers of the skin and mucous membranes. Innate immune responses are the immunity’s early warning system that is generated immediately after the encounter with a pathogen to prevent them from entering the body and to help eliminate the ones that have entered the body.

Once a microbe has entered the body, secretion of antimicrobial substances and activities of natural killer cells, phagocytes, the process of inflammation, etc. serve to eliminate it from the body. All these responses are the components of innate immunity.

answer: - few nutrients

high pressure

low temperatures

explanation: 1. few nutrients:   open-ocean zone is located way far from the land, which is the main source of the essential nutrients.2. high pressure: pressure increases by 1 atmosphere for every 10 meters increase in depth.3. ample sunlight: a large fraction of the sunlight is reflected back to the atmosphere from the sea surface. 4. varying salinity: below the thermocline, the water is isolated from the atmosphere so the salinity remains stable over the year.5. low temperatures:   the temperature of open-ocean zone ranges from a low of -2°c to an average of 17°c