Answer:
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Answer:
Having considered how an appropriate primary immune response is mounted to pathogens in both the peripheral lymphoid system and the mucosa-associated lymphoid tissues, we now turn to immunological memory, which is a feature of both compartments. Perhaps the most important consequence of an adaptive immune response is the establishment of a state of immunological memory. Immunological memory is the ability of the immune system to respond more rapidly and effectively to pathogens that have been encountered previously, and reflects the preexistence of a clonally expanded population of antigen-specific lymphocytes. Memory responses, which are called secondary, tertiary, and so on, depending on the number of exposures to antigen, also differ qualitatively from primary responses. This is particularly clear in the case of the antibody response, where the characteristics of antibodies produced in secondary and subsequent responses are distinct from those produced in the primary response to the same antigen. Memory T-cell responses have been harder to study, but can also be distinguished from the responses of naive or effector T cells. The principal focus of this section will be the altered character of memory responses, although we will also discuss emerging explanations of how immunological memory persists after exposure to antigen. A long-standing debate about whether specific memory is maintained by distinct populations of long-lived memory cells that can persist without residual antigen, or by lymphocytes that are under perpetual stimulation by residual antigen, appears to have been settled in favor of the former hypothesis.
A decrease in the overall volume of gases like hydrogen can prevent nuclear fusion in a nebula.
Explanation:
Nuclear fusion: The reaction of combining two or more atomic nuclei to form one or more different atomic nuclei and subatomic particles is known as nuclear fusion. A single heavy nucleus is formed from two lighter nuclei. Helium is made by fusing Hydrogen atoms together.
Nebula: A nebula is a giant cloud dust and gas in space. Nebulae are formed out of the gas and dust that are thrown out by the explosion of a dying star.
A decrease in the overall volume of gases like hydrogen can prevent nuclear fusion in a nebula.
