The thymus and bone marrow are.....

In the given figure,the part labeled as '$a$' represents:

Identify the function of $IgG$ antibody.

The principle of immunization or vaccination is based on the property of ..........

Memory cells ............

Person '$A$' cannot step out of his house. He has to spend his entire life in sterile isolation; otherwise, he would quickly contract a fatal infection. This person has almost no effective immune system. This disease is also called '$baby$ $in$ $a$ $bubble$ $syndrome$'. Identify the disease this person '$A$' is suffering from.