IMMUNITY is a biological phenomenon, which consists in a long-term autosupport of the genetic "self" and "not-self" balance in the body under foreign surroundings.
IMMUNE SYSTEM develops and releases certain mechanisms to exert the phenomenon. Understanding the basis of IMMUNOLOGY is achieved by the study both of the protective mechanisms challenged by "not-self" and of regulatory processes, which form internal homeostasis of "self" in the body.
Immunity may be innate (or nonspecific) and adaptive (acquired or specific). Paralysis of one component either some components of immunity may lead to immunodeficiencies and loss of host defense against infections, or tumours. Dysregulation of the immune system may result in autoimmune diseases, allergies (based on hypersensitivity of four types) and tumours. The importance of the immune system is illustrated by appearance about twenty years ago of a new disease - AIDS (acquired immunodeficiency syndrome) in which all the immune disorders may occur.

ANTIGEN (Ag) is a macromolecule, which contains a foreign or autologous information to launch immune responses. On the other hand, any antigen can be often used by explorers as an immunobiological marker. An appreciable account of various antigens is about 10¹⁸.

ANTIBODY or IMMUNOGLOBULIN (Ig) is a molecule of the immune system. Antibodies as well as receptors can be bound to appropriate antigens.

LYMPHOCYTES are major cells of the immune system. In fact, the immune system is a hierarchic totality of lymphoid cells (10¹³). There are T cells and B cells. Other cells (e.g. macrophages, dendritic cells, neutrophils, mast cells, etc) also attend many immune reactions.

Let us take functions of the immune system. The strategical function of the system is to switch genetic programme of ontogenesis, which goes ahead since birth till death under alien surroundings.

Tactical functions of the immune system:
 

The immune system functions as a partner of the central nervous system, the endocrine system and the liver very close to regulate the homeostasis.

There are two groups of mechanisms by which the immune system exerts its functional activity.

I. INNATE MECHANISMS, including NONSPECIFIC RESISTANCE: Resisting barriers: the skin, ciliated epithelium, sebaceous and sudoriferous glands, digestive enzymes, etc. Normal microbial cover of the body. Liver (cytochrome P450 system). Complement. Phagocytosis. Interferons (IFNs), NK cells, NKT cells and gdT cells. Acute-phase reaction. Proinflammatory cytokines. Natural antibodies. Toll-like receptors (TLR). Antimicrobial peptides (defensins and cathelicidins). II. ADAPTIVE MECHANISMS, or SPECIFIC IMMUNE RESPONSES: HUMORAL, or B-cell-mediated immune responses lead to production of plasma cells, which release the immunoglobulins (IgM, IgG, IgA, IgE, IgD). CELLULAR or T-cell-mediated immune responses result in formation of the effector T cells: 1. Cytotoxic T cells (or killer T cells). 2. Effector T cells of the immune inflammation. In innate immunity the immune system can recognize native "not-self" whereas in adaptive cases it always recognizes the processed "not-self" only that takes some time. Innate immunity is completed in non-clonal form. As contrasted to the immune responses, immunological tolerance is the state of specific unresponsiveness to an antigen.

Functional organization of the immune system may be considered at organic, cellular and molecular levels. There are two types of the immune system organs, central (or primary) and peripheral (or secondary). BONE MARROW is a central organ where all the immune cells are born and only B cells mature (process termed as B lymphopoiesis). THYMUS is the other central organ in which T cells mature (T lymphopoiesis). This organ also masters the whole immune system.

In the peripheral organs lymphocytes may be definitively matured after the interaction to antigens as a result of specific immune responses. The peripheral organs are:

• Lymph nodes, lymphatics and spleen.
• Mucous associated lymphoid tissue (MALT) including tonsils, Peyer's patches, isolated follicles, appendix, etc. In total, MALT may be divided in some levels:
  Eusthachian tube - TALT, nasal - NALT, bronchus (including mammary glands in females) - BALT, gut - GALT (small and large intestines).
• Skin associated lymphoid tissue (SALT).

Cells of the immune system may be divided into four functional groups:
 

There are some peculiarities of lymphocytes:

 

CLONE is a lymphocyte population committed to a certain antigen. It is considered there are dozens million B-cell clones and T-cell clones in each human organism. A committed lymphocyte involved in immune response becomes primed lymphocyte.

CD nomenclature ("cluster of differentiation") based on monoclonal technology allows specifying the cells concerning their origin, differentiation stage, activated state, etc.

MAJOR CD MARKERS

Cluster designation	Cells
CD10, CD34	Lymphoid stem cell
CD3	T cell
CD4	Helper/inducer T cell
CD8	Cytotoxic T cell
CD19, CD72, CD79, etc	B cell
CD16/CD56	NK cell
CD14, CD64	Monocyte/macrophage

There are lots of molecules, which are engaged in immune processes. All the molecules of the immune system must be classified.

SPECIFIC IMMUNE RESPONSE'S MOLECULES are unique substances characteristic for each clone and each specific process:

• Antigen-recognizing immunoglobulin receptors of B cells (BCR).
• Soluble immunoglobulins: IgM, IgG, IgA, IgE, IgD.
• Antigen-recognizing T-cell receptors ( TCR).
• Soluble fragments of TCR (Transfer Factors).
• Antigen-presenting molecules: human leukocyte antigens of histocompatibility (HLA)
  and CD1 (a, b, c, d and e).

PATTERN REGOGNITION RECEPTORS take part in innate immunity:

• Secreted molecules.
• Phagocytosis receptors.
• Toll-like receptors (TLR1-TLR11).

ADHESION MOLECULES provide interactions between immune cells and ligands by means of direct contact:

• Immunoglobulin superfamily.
• Integrins.
• Selectins.
• Mucin-like molecules, or vascular addressins.
• TNF/NGF superfamily.
• Link family.
• Cadherins.

IMMUNOCYTOKINES are immune system's hormones, which can regulate the relationships between the cells nearby and a long way off:

• Interleukins (ILs).
• Colony-stimulating factors (CSFs).
• Interferons (IFNs).
• Tumour necrosis factors (TNFs).
• Chemokines
• TGF superfamily.

VARIOUS MEDIATORS OF THE IMMUNE INFLAMMATION:

• Complement proteins.
• "Acute-phase" proteins.
• Prostanoids and leukotrienes.
• Free oxygen radicals, etc.