How immunity is acquired during a vertebrate's life cycle?

This depends on the type of immunity involved. Active immunity for example is acquired by exposure to the antigen either naturally or injections with the antigen. Passive immunity on the other hand is acquired by acquiring antibodies either naturally via the placenta form the mother or artificially trough injecting the antibodies to the body.

Acquired immunity is the adaptive immunity of a vertebrate. When the vertebrate is exposed to a pathogen, it fights the invader and retains specific memory of the pathogen such that it will eliminate it efficiently in next infection. If this occurs with every infection that the body is able to fight and recover, the immune system will have immunity to various diseases. Adaptive memory can also be induced before infection through vaccination.