Role of Complement In The Immune Response?

Complement components have immunologic activity both individually and in an activation cascade leading to a polymer formed by C5, C6, C7, C8, and C9 (the membrane attack complex, or MAC), which results in lysis of target cell membrane.

Early classic complement components (especially C3 products) act as opsonins and assist in the phagocytosis of bacterial particles by neutrophils and macrophages.

Certain complement split products (C3a and C5a) are chemotactic for phagocytic neutrophils and also act as “anaphylatoxins,” which directly stimulate mast cells and basophils to release histamine resulting in increasedvascular permeability.

Deficiency of early complement components is associated with increased pyogenic infections (C3 deficiency) and an increased incidence of autoimmune diseases, possibly owing to impaired clearance of immune complexes. The MAC appears especially important in host defense against Neisseria infection. Deficiency of any one of the terminal complement components can result in recurrent infections with Neisseria.

The complement system can be activated by three pathways:

Classical-IgM and IgG binding to antigen forming immune complexes that can bind Clq activating Clr and Cls to cleave c4. other proteins including c-reactive protein (binds Clq), serum amyloid P, and C4 nephritic factor can activate this pathway.

Alternative-activated by lipopolysaccharide on microbial cell surfaces in the absence of antibody. C3 and factor B bind to cell surface forming C3bBb, which functions to cleave more C3 molecules. This is part of the innare immune system. IgA complexes and C3 nephritic factor can also activate this pathway.

Lectin-mannan-binding lectin is secreted by the liver and binds to microbial ligands. This activates mannan-binding lectin-associated proteases that are related to Clr and Cls and can cleave C4 resulting in complement activation.

Neutrophils and Eosinophils In Immune Response

Neutrophil granulocytes, generally referred to as neutrophils, are the most abundant type of white blood cells in mammals and form an essential part of the immune system. Neutrophils are important in phagocytosing and digesting foreign particles at sites of inflammation and antigen entry. Neutrophils kill and dissolve microbes by

• Release of enzymes and bactericidal products from their intracytoplasmic granules
• By generation of toxic oxygen radicals and hypohalous acids.

Clinical deficiency of leukocytes manifests as recurrent skin and soft tissue infections with pyogenic organism and sepsis. Read more

Antigen Presenting Cells and T Cell Activation

Antigen-presenting cells are cells that express surface MHC (Major histocompatibility complex) class II molecules.  MHC class II molecules preferentially bind to T cell receptors associated with the CD4 surface molecule. Thus, APCs present antigen to the CD4+ T cells, the helper/inducer subset.

Class I MHC molecules preferentially bind to T cell receptors associated with the CD8 surface molecule.  Class I MHC molecules are present on the surface of all nucleated cells, thus allowing cells to present their internal antigens to cytotoxic T cells. Read more

A Short Note On Tuberculosis

About the Disease

Tuberculosis (TB) is a contagious disease caused by mycobacterium tuberculosis. When this bacillus is stained with carbol fuchsin it appears purplish red. The bacterium retains this dye even when it is washed with acid and alcohol. That is why it is called acid fast bacillus (AFB). Mycobacterium tuberculosis can remain dormant for many years in the lung without producing disease.

The spread of infection

The patient with pulmonary TB is the source of infection to others around him, especially when his sputum contains AFB. When such a patient coughs, he expels thousands of tiny droplets around him which have AFB in them.

Transmission of infection takes place when those around him breathe this contaminated air. Direct sunlight rapidly destroys AFB, but they can survive for long periods in darkness. Thus the risk of transmission of infection is highest in those persons who stay indoors with such a patient for long periods, e.g. family contacts. Read more

What Are Different Types of Antibodies

Depending on their structre and function there are five types of antibodies

IgG – Highest concentration in serum and excellent penetration into tissues. Can cross the placenta by week 16 of pregnancy. Fixes complement.

IgA – Most important antibody for host defense at mucosal surfaces (sites of antigen entry). Produced locally and often present in a modified form in secretions such as tears and salvia (secretory IgA). Secretory IgA is more resistant to enzymatic degradation. Read more

What Are Antibodies And How They Help In Body Defence?

Antibodies are also known as immunoglobulins and abbreviated as  Ig. These  are gamma globulin proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses.

They are typically made of basic structural units—each with two large heavy chains and two small light chains.

Antibodies are produced by a kind of white blood cell called a plasma cell.

There are several different different kinds of antibodies which are grouped into different isotypes based on which heavy chain they possess.

Five different antibody isotypes are known in humans and other mammals, which perform different roles, and help direct the appropriate immune response for each different type of foreign object they encounter.

Although the general structure of all antibodies is very similar, a small region at the tip of the protein is extremely variable, allowing millions of antibodies with slightly different tip structures exist. These tips are known as antigen binding sites. also  known as the hypervariable region.

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