(i) The human immunodeficiency virus belongs to the genus Lentivirus
(ii) When observed under the microscope, HIV is seen as a electron spherical virus, 100-12 nm in diameter, containing a dense core surrounded by a lipoprotein envelope.

(iii) The envelope has glycoprotein (gp) spikes termed gp 41 and gp 120. At the core, there are two large single stranded RNA.
(iv) Attached to the RNA are molecules of reverse transcriptase. It also contains enzymes like protease and ribonuclease.
(v) The core is covered by a capsid made of proteins. This is followed by another layer of matrix' proteins as shown in the Fig.

The different types of antigen-antibody reaction are:
(i) Precipitin: The reaction between soluble antigen and antibody leads to visible precipitate formation, which is called precipitin reaction. Antibodies that bring about precipitate formation on reacting with antigens are called as precipitins.
(ii) Agglutinstion: Whenever a particulate antigen interacts with its antibody, it would result in clumping or agglutination of the particulate antigen, which is called agglutination reaction. The antibody involved in bringing about agglutination reaction is called agglutinin.
(iii) Opsonisation: Opsonisation or enhanced attachment is the process by which a pathogen is marked of ingestion and destruction by a phagocyte. Opsonisation involves the binding of an opsonin i.e., antibody, to a receptor on the pathogen's cell membrane. After opsonin binds to the membrane, phagocytesare attracted to the pathogen. So, opsonisation is a process in which pathogens are coated with a substance called an opsonin, marking the pathogen out for destruction by the immune system. This results in a much more efficient phagocytosis.
(iv) Neutralization: The neutralization reactions are the reactions or antigenantibody that involve the climinai ion of harmful effects of bacterial exotoxins or a virus by specific antibodies. These neutralizing substances i.e., antibodies are known as antitoxins. This specific antibody is produced by a host cell in response to a bacterial exotoxin or corresponding toxoid.

Thymus is most active during the neonatal an pre-adolescent periods. The thymus is a flat and bilobed organ located behind the sternum, above the heart.
(ii) Each lobe of the thymus contains numerous lobules, separated from each other by connective tissue called septa. Each lobule is differentiated into an outer cortex and an inner medulla.
(iii) Within each lobule, the developing T cells called thymocytes are arranged into outer cortex and inner medulla. The inner medulla contains immature thymocytes and outer cortex has matured thymocytes.
(iv) One of its main secretions is the hormone thymosin. It stimulates the T cell to become mature and immunocompetent. By the early teens, the thymus begins to atrophy and is replaced by adipose tissue.