An continued Introduction to the elementary Setup of our Immune System #2

Image Source

The Adaptive Immune System provides vertebrates with a defense mechanism against foreign parasites such as viruses and bacteria. Its three main properties are:

• specific recognition of foreign molecules
• ability to destroy the foreign parasite
• memory mechanism that allows a more rapid response in the case of a second infection by the same pathogen

The sites on foreign molecules that are recognized by the immune system are called antigenic determinants. At this point the following terms shall be clarified:

An Antigen ... is any type of distinctive molecule or molecular structural element, which is capable of inducing an immune response in a host organism, hence is recognized by it's immune system.
An Antibody ... recognizes unique molecules, characteristic for pathogens (the Antigens) and therefore provides the immune system with the ability to distinguish between "self-" and "non-self-cells".

Representation of red and white blood cells - Image Source

Antigenic determinants interact with two different classes of antigen receptors produced by two major cell types - collectively called lymphocytes - of the immune system:

• T lymphocytes (or T cells) guard against virally infected cells, fungi, parasites and foreign tissue, hence providing "cellular immunity"
• B lymphocytes (or B cells) are most effective against bacterial infections and the extracellular phases of viral infections, hence providing "humoral immunity"

Lymphocytes are certain types of white blood cells, which are derived from stem cells in the bone marrow. In contrast to red blood cells, they can leave the blood vessels and patrol the intercellular spaces for invading pathogens.

On the Immunologic Memory:

On the function of B cells - a clonal selection theory

B cells rearrange the genes that code for their antibody proteins, so that each cell makes a unique antibody, also called "Immunoglobulin". Those antibodies are displayed on their cell surface in a membrane-bound form. According to the clonal selection theory, antigen binding to its complementary antibody on a specific B cell triggers rapid division and results in secretion of large amounts of the antibody specific to the original antigen.
This process is illustrated in the following image:

Antibody tagged pathogens are then recognized and disposed by macrophages and other effector cells of the immune system.

Amazing fact:
About 50 million B-lymphocytes with a unique antibody are produced by our body every day!

On the modus operandi of T cells

In contrast to B lymphocytes, T cells recognize antigenic determinants only when presented as part of a complex with a MHC protein. MHC means "major histocompatibility complex", and describes a region originally identified as the genetic element that controls transplant rejection. The MHC is remarkably polymorph and is "a marker of individuality".

MHC binds degraded fragments of peptide antigens, generated inside infected cells and display the fragment at the cell surface for recognition by T cells. Similar to B cells, T cells rearrange the receptor genes so that each T cell carries a unique T cell receptor. Binding of a T cell receptor to an MHC-antigen complex triggers activation of the T cell to kill the infected cell or elicits help from other cells in generating an immune response.