Question

An individual finds himself infected by a previously undiscovered pathogen, Transmissio.

Mention through what conduit soluble or mobile immune factors reach and interact with the pathogen.

Answer 1

The acquired immunity is primarily dependent on the action of the lymphocytes, B cells, or T cells. B cells and T cells have antigen receptors on their surface which are antigen-specific. Specific receptors are present on each lymphocyte. The immune responses of B-cells and T-cells after the infection are explained below.

The Ig (immunoglobulin) is the antigen-recognition molecules of the B cells. In adaptive immunity, the main function of the B cells is to secrete the antibodies that bind to the pathogens or the toxic products produced by the pathogens in the extracellular space of the body.

The T cells perform dual specificity. They identify the peptide antigens that are displayed by the MHC molecules and the self-MHC I or MHC II (major histocompatibility complex) molecules.

The cell recognition proteins recognize the foreign molecules or pathogens that invade the body and helps in improving the immunity of the organism. The major histocompatibility complex (MHC) glycoproteins are the cell recognition proteins.

The helper T cells (T_H cells) and cytotoxic T cells (T_C cells) are the two main types of T cells. TCR (T-cell receptor) that can identify a fragment of an antigen in combination with an MHC (major histocompatibility complex) molecule is present on each type of T cells.

T cells need help in recognizing an antigen. The APC (antigen-presenting cell) such as macrophage or dendritic cell presents or displays the antigen to the T cells. After destroying and ingesting into a pathogen, APCs move to the spleen or a lymph node, where T cells also get collected. After rupturing the pathogen with lysosomal enzymes, a fragment of the pathogen is presented by the APC in the groove of the major histocompatibility complex (MHC) molecule on its surface. Here, MHC combination or the antigen fragment is recognized by the T cell. MHC molecules or proteins are also involved in recognizing foreign tissues. Thus, before organ transplantation or tissue transplantation, an attempt is made to find an organ donor whose MHC proteins match with the recipient as closely as possible because T_C cells are able to recognize the foreign MHC antigens.

The cytotoxic T cells (T_C cells) contain storage vacuoles that have many perforin molecules and enzymes known as granzymes. The perforin molecules are released when T_C cells get activated when it binds to a cancer cell or virus-infected cell that presents foreign antigen in the groove of the MHC class I molecules present on its surface. The pores are formed on the abnormal cells by the perforin molecules. The granzymes now enter the cell that is targeted that dies after undergoing apoptosis.

The white blood cells, lymphocytes of the adaptive immunity have evolved to identify and fight against the broad range of pathogens. The immunoglobulins or Ig are the antigen-recognition molecules of the B cells. B cells produce these proteins in a wide range of the antigen specificities, each immunoglobulin produced by the B cells have a single specificity. The BCR (B-cell receptor) is the membrane-bound immunoglobulin present on the surface of the B cell that serves as a receptor for the antigen. The terminally differentiated B cells, plasma cells secrete the immunoglobulin of the same antigen as an antibody. In adaptive immunity, the main function of the B cells is to secrete the antibodies that bind to the pathogens or the toxic products produced by the pathogens in the extracellular space of the body.

The defense provided by the B cells is known as antibody-mediated immunity because the activated B cells become plasma cells that form antibodies. It is also known as humoral immunity because the antibodies are found in the lymph or blood.

The B cells activate in the spleen or a lymph node when the specific antigen binds to its specific B-cell receptor (BCR). The B cells undergo clonal expansion and divide by mitosis several times soon after antigen binds to its BCR. B cells form many copies of itself and most of the clones transform into or become plasma cells that circulate in the lymph and the blood.

Few cloned cells are transformed into memory cells which are critically important for long-term immunity. The second exposure of any antigen results in the quick division of the memory B cells that further give rise to more plasma cells that have the ability to produce the correct antibody rapidly.

The plasma cells are greater than regular B cells because of the presence of an extensive network of RER (rough endoplasmic reticulum) for the production and secretion of the antibodies to a particular antigen on a larger scale.

Therefore, B cells generate the plasma cells that produce the antibodies which combine with a particular antigen and neutralize it.

In T cells, the antigen recognition molecules are made of membrane-bound proteins and perform a single function of signaling the activation of T cells. The TCRs (T-cell receptors) are similar to the immunoglobulins both in their genetic mechanism to produce great variability and in the structure of their protein having both C and V regions. However, TCRs do not identify and attach to the antigen directly, but instead identifies the short fragments of the pathogen’s protein antigens peptide which are attached to the MHC molecules on the other cells surfaces.

The major difference between the B cells and T cells is that in the viruses and bacteria, B cells can identify the surface antigens whereas T cells can identify only the bacterial or viral antigens present outside the infected cell.  

The complement system or the complement consists of a number of blood plasma proteins. Certain immune responses are complemented by this system. The strong and efficient proteolytic cascades are formed when the complement system is activated.

Generally, the complement system works along with the innate immune system. It behaves as an innate defense against the pathogens. The strong and efficient proteolytic cascades are formed when the complement system is activated that results in the generation of the inflammatory response by the production of the effective pro-inflammatory molecules and causes the lysis of the pathogen molecules after opsonization.

The complement system is a system comprising of plasma proteins that can be activated by the direct invasion of the pathogens or indirectly by the pathogen-bound antibody that results in a cascade of reactions that happens on the pathogen’s surface and active components with the effector functions are generated.

A primary response is generated after the first exposure to a foreign antigen (pathogen). For a few days, antibodies are not present, then there is a rise in the antibodies. A secondary response is generated after the second exposure to the pathogen. The immune response is extremely high at the time of the second exposure, thus, the second exposure is also called a booster. The disease symptoms are prevented even after the exposure of the individual to the disease-causing agent because the antibodies and the antigen-specific T cells are extremely high for a longer period of time in active immunity after the second exposure.

The diagrammatic representation of the complement system functioning is given below:-