Immune System Defenses: Types and Processes
Nonspecific Defenses
The immune system employs various nonspecific defenses to protect the body from pathogens.
Phagocytes
Phagocytes are leukocytes responsible for removing microorganisms and foreign substances from tissues through phagocytosis.
Phagocytes move via amoeboid movements.
Types of Phagocytes
- Granulocytes: These cells contain abundant granules with enzymes and antimicrobial compounds.
- Monocytes: These cells lack granules and have a lobulated nucleus. They are larger than granulocytes.
Phagocytic Process Steps
- Union: Attachment to the item to be phagocytosed. This is facilitated by opsonins.
- Ingestion: Engulfment via pseudopodia.
- Digestion: Death and intracellular digestion of the pathogen.
- Expulsion: Expulsion of undigested remains.
Initially, monocytes present within the invaded tissue cells with limited capacity act as phagocytes. Subsequently, neutrophils arrive from the blood, but they are short-lived. Finally, macrophages appear. After phagocytosing a certain amount of bacteria and organic debris, the phagocytes die. The collection of phagocytes and the remains of dead organisms form pus, which can be absorbed or expelled.
Complement System
The complement system comprises approximately 30 proteins present in blood plasma. Its defensive function (both nonspecific and specific) develops rapidly and complements and enhances the action of antibodies.
It performs three main functions:
- Mediator of inflammation.
- Facilitates phagocytosis and the action of antibodies.
- Causes the breakdown of the plasma membrane of invading cells.
Interferon
Virus-infected cells release proteins called interferons, which prevent the infection from spreading by acting in two ways:
- Prevents replication of the virus in infected cells that have not yet been destroyed.
- Activates leukocytes that can recognize virus-infected cells or cancer cells and destroy them.
Interferon is species-specific, meaning it has protective action against viruses that can invade a specific species but not in the cells of another species.
Humoral Response
The humoral response involves the production of antibodies by B lymphocytes.
When an antigen is encountered, antibody binding occurs to a membrane of a specific B lymphocyte.
After antigen recognition, B cells are activated. Their activation leads to the rapid division of these cells, producing an equal number of cells, all producing the same type of antibody.
Some activated B cells remain as memory B cells, which have an unlimited lifespan and serve as a reserve for future exposures to the same antigen.
The first time the antigen is recognized is called the primary immune response. When the same antigen re-enters the body and is recognized by memory cells, the secondary response quickly starts, being faster than the primary response, and memory cells rapidly divide into plasma cells.
Antibodies
Antibodies are proteins with a carbohydrate fraction.
Each antibody molecule consists of four chains: two larger (heavy chains) and two smaller (light chains). Both heavy chains and both light chains are identical.
Disulfide bridges exist between heavy chains and between heavy and light chains, giving the molecule a Y-shaped structure.
Each antibody molecule has a constant region that is the same for each type of antibody but different between them. This constant region is responsible for binding to the membrane of B cells, phagocytes, or complement.
There is a variable region at the end of the arms of the Y, where the amino polypeptide chain and the union with the antigen occur.
Function of Antibodies
Antibodies recognize pathogens by binding to antigens present on their surface. The formation of the antigen-antibody complex results in direct and indirect effects:
- Neutralization
- Precipitation
- Agglutination
- Ag-Ab complexes stimulate the action of phagocytes by opsonization.
- The Ag-Ab binding facilitates the activation of complement.