Bursa of Fabricius of birds, bone marrow and thymus gland of mammals are the primary lymphoid organs.

Normal cells show a property called contact inhibition, which inhibits uncontrolled growth. Cancer cells do not have this property. As a result, cancerous cells divide continuously giving rise to mass of tissues called tumours.

It is a test which is more reliable and is a confirmatory test to detect the presence of viral core proteins in the case of HIV infection.

A vaccine is a biological preparation that provides active acquired immunity to a particular disease and resembles a disease causing microorganism and is often made from weakened or attenuated or killed forms of the microbes, their toxins, or one of its surface proteins.

The binding force between antigen and antibody is due to three factors. They are
(i) closeness between antigen and antibody
(ii) non-covalent bonds or intermolecular forces and
(iii) affinity of antibody.

They functions as antibodies. The functions of immunoglobulin are agglutination, precipitation, opsonisation, neutralization etc.,

Haptens are substance that are nonimmunogenic but can react with the products of a specific immune response.

Killer cells are type of T lymphocytes which move around the body and destroy cells which are damaged or infected.

Helper T cells are a type of T lymphocytes and release a chemical called cytokine which activates B lymphocyte.

Inflammatory barriers is a type of Innate Immunity. Tissue damage and infection induce leakage of vascular fluid, containing chemotactic signals like serotonin, histamine and prostaglandins.

Interferons are a group of signalling proteins produced by virus infected cells. They interfere with viral replication and hence named interferons. A virus infected cell releases interferons which induce antiviral state in the uninfected cells. They are chemical mediators under physiological barriers offering Innate immunity.

The type of barrier that involves macrophages are the
(i) Phagocytic barriers which is a type of innate immunity.
(ii) In this mechanism, specialised cells such as monocytes, Neutrophils and tissue macro phages pha cytose and digest whole microorganisms.

The immune responses may be primary or secondary.
(i) The primary immune response occurs when a pathogen comes in contact with the immune system for the first time.
(ii) The secondary immune response occurs when a person is exposed to the same antigen again. During this time, immunological memory has been established and the immune system can start producing antibodies immediately. Within hours after recognition of the antigen, a new army of plasma cells are generated. Within 2 to 3 days, the antibody concentration in the blood rises steeply to reach much higher level than primary response. This is also called as "booster response".

Lysozyme present in saliva acts as an antibacterial agent and cleaves the bacterial cell wall.

(i) Primary Lymphoid organ: Thymus is a primary lymphoid organ.
Function: One of its main secretions is the hormone thymosin. It stimulates the T cell to become mature and immunocompetent.
(ii) Secondary Lymphoid organ: Tonsils is a secondary lymphoid organ.
The tonsils are part of the lymphatic system, which help to fight infections. They stop invading germs including bacteria and viruses.