Immunity is the ability of the body to resist pathogens that cause damage to tissues or organs through biological defense. Leukocytes play a major role in providing immunity.
## Active Immunity
This is a type of immunity in which the body develops either antibodies or activated T cells in response to the invasion of the body by a foreign antigen. They are two types of active immunity namely; Innate & Acquired immunity
## Innate Immunity
Is also known as native immunity. It is the resistance to infection that an individual possesses by virtue of his/her genetic and constitutional makeup. They are triggered by receptors that are coded in the germline. They provide the first line of defense against infections. It triggers the slower but more specific acquired immune response. Are of two types;
* **Humoral:** mediated by tears, saliva, mucous membrane, skin, stomach acid, lysozymes, and interferons.
* **Cellular:** is an inborn immunity developed by cell-mediated response carried out by neutrophils, eosinophils, basophils, macrophages, and natural killer cells.
## Mechanism Of Innate Immunity
* Microbial antagonism
* Inflammation
* Fever
* Antibacterial substance in blood and tissue
* Cellular factor in innate immunity
* Epithelial surfaces
## Acquired Immunity
Also known as adaptive immunity. It is the resistance that human beings acquire during life. It is triggered/initiated by antigens. Lymphocytes are responsible for acquired immunity. Are of two types namely Humoral & Cellular immunity
## Humoral Immunity Introduction
It is a type of immunity mediated by antibodies secreted by B-lymphocytes into the blood and lymph (humors). It is the main defense mechanism against bacterial infections i.e. they mainly attack bacterial cells.
### B-lymphocytes (introduction)
They are responsible for humoral immunity. This population of cells was first discovered in birds which have a processing organ known as the bursa of fabricus.
**Sites of processing**
* In mid-fetal life – liver
* In late fetal life & after birth – bone marrow
### Sites Of Storage
Secondary lymphoid organs such as lymph nodes, spleen, and tonsils (throat)
### Types & Functions
* **Plasma cells:** They destroy the foreign organisms by producing antibodies. The antibodies they produced are released into the lymph and transported into the circulation and produced until the end of the lifespan of the plasma cells.
### Antibodies (Immunoglobulins)
The antibodies are gamma globulins called immunoglobulins. Immunoglobulins are composed of light and heavy chains of polypeptide. They have a molecular weight of between 160,000 – 970,000. They contribute about 20% of plasma proteins.
**TYPES OF ANTIBODIES**
* IgG
* IgA
* IgM
* IgE
* IgD
### The Complement System
Is a system of about 20 proteins many of which are enzyme precursors. Proteins include C1 through to C9, factors B, D, P & regulatory proteins. They provide a major mechanism for destroying foreign substances in the body. They kill bacteria & other types of cells (N.B – our cells are immune to the complement proteins). They have two main pathways; classical and alternative pathways
### Effects Of The Complement System
* **Opsonization & phagocytosis:** – C3b activates phagocytosis by both neutrophils and macrophages
* **Agglutination:** – the complement product changes the surface of the invading organism causing them to adhere to one another
* **Neutralization of viruses:** – they attack the structures of the viruses and render them non-virulent
* **Lysis:** – C5b6789 forms a membrane attack complex which has a direct effect of rupturing the cells of bacteria and other organisms
* **Chemotaxis:** – C5a initiates the migration of a large number of phagocytes to the site of inflammation
* **Activation of Mast cells & Basophils:** – C3a, C4a & C5a activate mast cells and basophils causing them to release histamine, heparin, and other substances into the local fluid
* **Inflammatory effect:** – the products of the complement system cause the increase of the already increased blood flow, and increased capillary leakage of proteins
* **Memory B-cells:** They are inactive lymphoblasts that get activated upon second exposure to the same antigen
## Cellular Immunity Introduction
It is a type of immunity mediated by the T-lymphocytes. It is the major defense mechanism of the body against infection by viruses, fungi & few bacteria. Is also responsible for the late allergic reactions.
**Sites of processing of T-lymphocytes**
* Thymus
**Sites of storage**
* Lymphoid tissues such as lymph node, bone marrow, GIT
### Types & Functions Of T-Lymphocytes
* **Cytotoxic T-cells:** – AKA Killer cells. They are direct attack cells capable of killing microorganisms by secreting hole-forming proteins called perforins.
* **Helper T-cells:** – are the most numerous T-cells and help in the functions the immune system by forming a series of protein mediators called lymphokines
* **Suppressor T-cells:** – it regulates the action of both Helper T-cells and Cytotoxic T-cells from causing autoimmunity
* **Memory T-cells:** – they are inactive lymphoblasts that get activated upon exposure second to the same antigen
## Passive Immunity
This is the transfer of active humoral immunity of ready-made antibodies from one person (or animal) to another.
**Autoimmune diseases:** a number of disorders caused by a reaction of the immune system against the body’s own cells and tissues.
* Neutrophil hypomotility syndrome
* Chronic granulomatous diseases
* Congenital myeloperoxidase deficiency
