Mechanism of action of T-lymphocytes- Part I: T-Helper Cells

Introduction

I would like to bring few basic concepts of immunology in my subsequent blogs. To start with I would like to discuss with everyone about mechanism of action of T-helper cells (T-helper lymphocytes). Though, the role played by T-helper cells is varied and wide, but the stimulus which makes T-helper cells to start acting is more or less the same. Actually, the domain of immunology is very complex to understand but the different types of lymphocytes start acting in their own unique ways.

The initial Stimulus

The initial impulse depends much upon the antigens against which the body has to act. These antigens may be bacterial antigen, xenobiotics (foreign substances) etc. These antigens are initially ingested by a special class of leucocytes named Antigen Presenting Cells (APC) which constitute primarily of macrophages and monocytes. These APC ingest by the way shown in the diagram below.

Moreover, these APC have a special components which aid in their recognition by the lymphocytes. These ‘components’ are called as Major Histocompatibility Complex- II (MHC-II). These processed antigens are then bound over to MHC-II over APC surface. This plays as an initial actor to stimulate T-helper cells later in the play.

 Structure of T-helper cells

 The T-Helper cells has a structure resembling the picturized diagram given below:

It consists of a regular eukaryotic leukocyte cells; the difference being primarily in the cell membrane molecules.

It has 2 main receptors:

1) TCR- This abbreviates for T-Cell Receptor and is the receptor that is present in all the lymphocytes and functions to bind the MHC classes in the body and thereby getting evoked to play further actions.

 2) CD4 – This is present only on T-helper cells and characterizes this sub-class of lymphocytes.  

Response by T-Helper cells

 Now, the APC is ready with the processed antigens. The only task that is remaining is to summon the T-Helper cells. The APC plays a role here by secreting various types of cytokines. These chemicals are chemo attractant in nature and allures the lymphocytes. But only T-Helper cells can bind because only it has the receptor to bind to MHC-II, that is present over APC. On binding of T-helper cells, they further evoke the phagocyte cells and B-cells (humoral immunity; might accompany activation of complement system further amplifying the protective response and even damage to body’s own cells), by releasing lymphokines. These phagocytes include macrophages, monocytes and neutrophils. Neutrophils and monocytes play a role in 1st 24 hours of inflammatory response, while macrophages, being the activated form of monocytes (Monocytes converted macrophages), and dominate the aftermath of inflammatory response after 24 hours.

Cumulative Event

 To summarize the event, the below given diagram might help a lot!

Conclusion

This was a very minor portion of role played by T-Helper cells and disabling these cells in AIDS might explain you well what causes the whole of body’s immune system to get dampened down. There is no activation of phagocyte cells and hence, no neutralization and killing of toxins and microbes. I hope I would have convinced and satisfied you regarding presentation of this blog before you and I am really sure that it would ensure me that I have done my job if I receive your comments and discussions.

Reference Texts

1) Kuby Immunology

2) Abbas, Lichtman basic immunology