How do vaccines help the immune system fight against pathogens , like viruses and bacteria ?

It introduces the body to a virus strand(s) to initiate internal resistance, which is created after a variety of metabolical processes. The idea is that the body would develop immunity to the virus without actually "getting" the virus.

Explanation:

When we are exposed to antigens the body sends an immune response to prevent any infection or disease. Vaccines contain the same parts of antigens that can cause diseases, though they are weakened so that they don't actually cause disease, then they trigger the immune system to produce antibodies, which then help to fight against pathogens.

Explanation:

unicellular organisms, like bacteria, are able to perform all life functions within one single cell. multicellular organisms need many different types of cells to carry out the same life processes. each of these special types of cells has a different structure that it perform a specific function.

idk what the options are but

these is a strong electrostatic attraction between the ions hence a lot of energy is needed to overcome this attraction

this is why the boiling points are so high