Directions: Read the passage given below and answer the questions that follow by choosing the correct/most appropriate options:

The young human child, even as the innate and adaptive immune systems start to mature, is at risk from many pathogenic viruses, bacteria, fungi, and parasites. Nevertheless, he or she has a good chance of survival in developed countries. Before there was good nutrition, hygiene, and comprehensive vaccination, there was high mortality in infants and young children. In 1900, the UK infant mortality rate was 140 per 1000, falling to 7 per 1000 by 2000. This reduction in mortality was proportionally greater in infants and children compared with other age groups. Better prevention and control of infections account for most of this fall. However, in many countries, infant mortality rates remain above 50 per 1000, giving some indication of the evolutionary pressure that must have selected a working protective immune system. Furthermore, such pressure has selected the extreme genetic polymorphism in the MHC, which through peptide presentation to T cells and NK cells is a key regulator of almost all immune responses.

The immune system gradually matures during infancy. Critical early protection against many infectious diseases previously experienced by the mother is given by the passive IgG antibody transferred from the mother transplacentally and in milk. Once that fades away, young children become more vulnerable to infections, though by then better armed with the maturing innate and adaptive immune systems. The risks are now much reduced by vaccinations, which stimulate protective immune responses in the maturing immune system. Nevertheless, children may still acquire viral, bacterial, and parasitic infections that have to be fought off and controlled by immune responses. Besides promoting recovery, such antigen stimulation results in immunological memory. Thus, over time, protection provided by the immune response increases, and young adults suffer fewer infections. This accumulation of immunological memory is an evolving feature of the adaptive immune response. The memory persists into old age but then may fade.

Besides frank infections and vaccinations, the newborn is exposed to other antigens. He or she comes from a relatively sterile environment in utero and is then rapidly _______ to multiple microbes. The first major exposure to bacteria is during passage through the birth canal, and then as soon as he/she makes oral, skin, and respiratory contact with the exterior. From then on, exposure to microorganisms is continuous. the gut and other mucosal sites (1)/ including the digestion of food (2)/ and the acquisition of vital nutrients (3)/ are essential for a healthy life (4)/ many of the bacteria that colonize (5)/. They also impact the development of the immune system.

As the child grows, the immune repertoire is also shaped by intercurrent infections and vaccinations. Pathogenic infections can be documented by symptomatic illnesses suffered by the child or adult, but for many viruses, such as influenza, an infection may be subclinical, but still sufficient to stimulate or boost immune responses. Generally, the protection offered by the immune response, both by antibodies and T cells, is very potent. Most childhood infections happen only once and then protection is lifelong.

It is remarkable that a mother can transfer sufficient antibodies to protect her infant when she was infected 20–30 years previously. The transmission of protective antibody protection from a mother to her child is hugely important, especially in environments where 15% or more infants and children die of infection. Paradoxically, a mother who avoided a dangerous childhood infection, through herd immunity, may actually put her child at risk by being unable to transfer specific protective antibodies.