(i) After getting into the body of the person, the virus enters into macrophages where RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase.
(ii) This viral DNA gets incorporated into the DNA of host cells and directs the infected cells to produce viral particles.
(iii) The macrophages continue to produce viral particles and act like a HIV factory. simultaneously, HIV enters into helper T-lymphocytes (T_H), replicates and produces progenies viruses.
The progenies viruses are released into the blood attack other helper T-lymphocytes. This is repeated, leading to a progressive decrease in the number of helper T-lymphocytes in the body of the infected person.
(v) During this period, the person suffers from bouts of fever, diarrhea and weight loss. Due to decrease in the number of helper T-lymphocytes, the person starts suffering from infections that could have been otherwise overcome, such as those due to bacteria especially mycobacterium viruses; fungi and even parasites like toxoplasma.
(vi) The patient becomes so immune deficient that he/she is unable to protect himself/herself against infections.

(i) The primary lymphoid organ is thymus.
(ii) The secondary lymphoid organ is tonsils.
Role of Primary lymphoid organ: Thymus
(i) Thymus is most active during the neonatal and pre-adolescent periods.
(ii) The thymus is a flat and bilobed organ located behind the sternum, above the heart.
(iii) Each lobe of the thymus contains numerous lobules, separated from each other by connective tissue called septa (cross walls).
(iv) Each lobule is differentiate into an outer cortex and an inner medulla. Within each lobule, the developing T cells called thymocytes are arranged into an outer cortex and an inner medulla
(v) An inner medulla contains immature thymocytes and outer cortex has matured thymocyte
(vi) One of its main secretions is the hormone thymosin. It stimulates the T cells to become mature and immunocompetent. By the early teens, (upto 12-13 years) the thymus begins to atrophy and is replaced by adipose tissue (binding tissue).
Secondary lymphoid organ :
(i) The tonsils (palatine tonsils) are a pair of soft  tissue masses located at the back of the throat (pharynx ).
(ii) The tonsils are part of the lymphatic system which help to fight against the infections. They stop invading germs including bacterin and viruses.

(i) The types of barriers which involve macrophages are phagocytic barriers
(ii) It is also known as cellular barriers.
(iii) Certain types of leucocytes (WBCs), such as polymerase nuclear leukocytes (PMNL) : neutrophils, monocytes and natural killer cells (a type of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes.

(i) An infection or tissue injury usually causes redness (rubber), swelling (tumor), pain (dolor). and production of heat (color) that may result in fever
(ii) Such localised expression is called inflammatory response, which occur due to release of certain chemical alarm signals such as histamine and prostaglandins.

Interferons: These are the proteins made and released by host cells in response to the presence of pathogens such as viruses, bacteria, parasites etc
RoIe: They inhibit viral infections and stimulate the entire immune system to fight disease. They also regulate many kinds of cell functions