Question

In immunology:

1. What do we mean by recognition of self vs. non-self? What specifically does “recognition” mean in this context? What is the molecular nature of this interaction.
2. What is immunological memory? Why does it matter, and what specifically does it allow for that has absolutely saved billions of lives?
3. Why is it useful to have two very different mechanisms of antigen recognition (ie BCR vs TCR)? What problem might be more likely to occur if all Ag were recognized the same way?

Answer 1

1. Recognition of self vs non-self cells:-

In the human body there are a large number of protein present which are divided into classes. One such class is the MHC proteins also called as HLA antigens. MHC (Major Histocompatibility Complex) proteins are synthesized by the MHC genes present in the chromosome 6. They help in differentiate between the self and non-self cells and helps in its recognition. They are also known as Human Leukocyte antigens. The self cells are the cells that body recognizes as a part of it. The non-self cells are foreign cells which the body doesn't recognizes as its own.

Molecular nature of the interaction.

The MHC class of proteins are of two typs:-

a. MHC class I molecules- They are found in all types of body cells.

b. MHC class II molecules- They are present in macrophages, B lymphocytes, Langerhan's dendritic cells etc.

When the antigen enters the body, they may be intracellular in case of viruses or extracellular in case of bacteria. The antigens are processed by APC's (Antigen Presenting Cells) along with MHC class I molecules for viruses and the antigens are processed by B cells or macrophages along with MHC class II molecules for bacteria. After recognition and processing, the viral infected cells are presented to T8 cells and the bacterial infected cells are presented to T4 cells. CD4 and CD8 are proteins present with T4 and T8 types of T lymphocytes which has an active role in binding of MHC proteins with the T cell receptor bringing about the reqired immunological synapse.

2. Immunological Memory Cells and its Significance:-

The B lymphocytes are formed in the bone marrow and are responsible for the production of humoral immunity. Humoral immunity helps in the body defense against foreign antigens, through the production of immunoglobulins or antibodies. They also function as Antigen Presenting Cells.

The B lymphocytes are converted into plasma cells which produces immunoglobulins and memory B cells. The memory B cells functions in the recognition of foreign antigen in the subsequent infections with the production of more severe immunoglobulins. It forms a part of active immunity. The significance of memory B cells comes into the picture in the defense against the dangerous foreign pathogen infections which can be very fatal. During the first encounter with the pathogen, the immune response is less severe and thus the immunity takes time to counter the infection. After the immune response, some of the antibodies are converted to memory B cells. Upon subsequent infections, the memory B cells recognizes the antigens and helps in the more severe immunological response by producing a large number of antibodies and thus aiding in the quick recovery. This forms the basis of active immunity and the basis of vaccination programmes. This has helped in saving of the billion of lives across the globe.

3. The two different mechanisms of antigen recognition are B cell recognition (BCR) and T cell recognition (TCR):-

1. B cell recognition works through the production of antibodies in which there are mainly unprocessed antigens. The B cell recognition mechanism is less severe than the T cell recognition and is a type of innate immune response. It has a single specificity , i.e, it produces the same kind of antibodies in all kind of infections. It forms a basis of passive immunity.

2. T cell recognition works through the formation of the MHC complex and consists of processed antigens. The processed antigens are small fragments of linear polypeptide which has a higher specificity for each kind of infection. It forms a basis of non-innate immune response or active or adaptive immunity and has helped in saving of countless lives on this planet. The response is more severe and stronger than B cell recognition mechanism.

If all the antigens were to be recognized by the same mechanisms, there would occur, same type of immune responses which will not be specific and will not work for different pathogens causing different clinical manifestations. This will lead to ineffective immune response and will prove highly fatal for the patient.