IMMUNITY

Definition

Immunity is a state of responsiveness to foreign substances such as micro organisms and tumor proteins. Immune response has three functions.

• Defense

The body protects against invasions by micro organism and prevents the development of infection by attacking foreign antigens and pathogens

• Homeostasis

Damaged cellular substances are digested and removed. By this mechanism, the body’s different cell type remains uniform and unchanged.

• Surveillance

Mutations abnormality continually crisis in the body but it is normally recognized as foreign cells and destroyed.

TYPES OF IMMUNITY

  Immunity is classified as innate (natural) or acquired

• Innate Immunity: This exists in a person without any contact with an antigen (foreign body). This involves the response of Neutrophils and monocytes
• Acquired Immunity: This immunity is developed either actively or passively
  • Active acquired Immunity: This results from the invasion of the body by foreign substances such as micro organisms and then develops antibodies and sensitized lymphocytes. Active Immunity take several weeks to develop but they are Longlasting
    • Natural Active Acquired Immunity: When micro organism invades again the body responds more rapidly and vigorously to fight with the invading micro organisms. This immunity is obtained naturally when the person comes in contact with the disease
    • Artificial Active Acquired Immunity: when the immunity is obtained artificially through inoculation of a killed or less virulent antigens through Immunization (Vaccination)
  • Passive Acquired Immunity: This implies that the host received antibodies to an antigen rather by sensitization. In passive Immunity the Protection is Immediate, but their acts lasts for few weeks to months
    • Natural Acquired Passive Immunity This can be acquired by placental transfer of immunoglobulin to the fetus from the mother
    • Artificial Acquired Passive Immunity: This can be achieved through injecting gamma globulin (serum antibodies) But passive immunity is short lived because the body does not produce antibodies and do not retain the memory cell of antigen.

Lymphoid organs:

The lymphoid system is composed of central and peripheral lymphoid organs

• Central Lymphoid organs: Thymus gland and bone marrow
• Peripheral Lymphoid organs: Tonsils, gut, genital, bronchial and skin associated lymphoid tissues, lymph nodes and spleen

Lymphocytes are produced in the bone marrow and eventually migrate to the peripheral organs. The thymus is important in differentiation and maturation of T-lymphocytes and therefore essential for cell mediated immune response

Lymphocytes:

Lymphocytes are produced in the bone marrow. Lymphocytes differentiate in to Band T lymphocytes.

B-Lymphocytes: B-cells are formed in the bone marrow and differentiate into plasma cells when activated plasma cells produce antibodies (Immunoglobulins)

• T-Lymphocytes:

Cells that migrate from the bone marrow to the thymus differentiate into T-Lymphocytes (Thymus dependent cells). Thymus secretes hormones, (Thymosin) which stimulate the maturation and differentiation of T-lymphocytes. “T” cells are responsible for producing immunity to intracellular viruses, tumor and fungi

T-Lymphocytes can be categorized in to T-Cytotoxic and T-helper cells. All mature T-cells have CD3 antigens

• T-Cytotoxic cells

T-Cytotoxic (CD8 or cytolytic cells are involved in attacking antigens on the cell membrane of foreign pathogens and releasing cytolytic substances that destroy the pathogen. Some sensitized ‘T’ cells remain as memory cells.

• T-Helper cells

T-helper (CD4) cells are involved in regulation of cell mediated immunity and the humoral antibody response T cells differentiate into subset of cells that produce distinct types of cytokines. The subsets are called TH1 cells and TH2 cells.

TH1 cell stimulate phagocyte mediated ingestion and killing of microbes, the key component of cell mediated immunity.

TH2 cells stimulate phagocyte dependent, Eosinophils mediated immunity which is effective against parasites and allergic response.

• Natural Killer cells

These cells are also responsible in cell mediated immunity. These cells are not T or B cells but are large lymphocytes with numerous granules in the cytoplasm. Natural killer cells are involved in recognition and killing of virus infected cells tumor cells and transplanted cells.

Immune Response

There are two types of immune response in humans namely humoral and cell mediated immunity