HLA and Immune Response

I'm having a hard time differentiating between what phagocytes do and what antibodies do? I know phagocytes engulf the pathogen and present the antigen and T-killer cells destroy it. So what do antibodies do?

I'm trying to put this into the context of the HLA antigens on transplanted organs. I know the antigens are recognised as foreign/non-self but I'm not sure what detects them and what destroys the organ?

Thank you.

Reply 1

Tristian Fox

Antibodies are produced by the B Cells, which are complementary to those of the foreign cells. Antibodies have several different functions in the destruction of bacteria, including Agglutination and Lysis. The HLA antigens are those found on human cells and are different in each person give or take a few. In transplants the cells have antigens that are non-self, ie they match a antibody in that persons immune system. These foreign antigens are recognised by receptors on the phagocytes and are engulfed, the foreign antigen is then presented and a immune response is activated, which targets the cells in that organ due to their foreign HLA system. The more antigens two people have in common the less likely it is that this difference will be detected.

Hope this helps.

Reply 2

blossomx

OP

20

Original post by Tristian Fox

Antibodies are produced by the B Cells, which are complementary to those of the foreign cells. Antibodies have several different functions in the destruction of bacteria, including Agglutination and Lysis. The HLA antigens are those found on human cells and are different in each person give or take a few. In transplants the cells have antigens that are non-self, ie they match a antibody in that persons immune system. These foreign antigens are recognised by receptors on the phagocytes and are engulfed, the foreign antigen is then presented and a immune response is activated, which targets the cells in that organ due to their foreign HLA system. The more antigens two people have in common the less likely it is that this difference will be detected.

Hope this helps.

Thank you so much.
So just to confirm, the B lymphocytes don't detect the HLA antigens, it's the phagocytes that do. Then once the antigen is presented is when the B lymphocytes recognise it?

Reply 3

Tristian Fox

That's right, B Lymphocytes are activated by an APC (Antigen Presenting Cells), which are phagocytes that have detected, engulfed and presented the foreign antigens.

Reply 4

blossomx

OP

20

Original post by Tristian Fox

That's right, B Lymphocytes are activated by an APC (Antigen Presenting Cells), which are phagocytes that have detected, engulfed and presented the foreign antigens.

Thank you. :smile:

Reply 5

Svenjamin

17

Original post by Tristian Fox

That's right, B Lymphocytes are activated by an APC (Antigen Presenting Cells), which are phagocytes that have detected, engulfed and presented the foreign antigens.

Original post by LeaX

Thank you. :smile:

Sorry guys, but this is completely wrong

APCs do not activate B cells. The APCs activate T cells (via interactions between the HLA/antigen complex and TcR), and then the T cells in turn orchestrate the adaptive immune response, such as activating B cells. There is never any direct interaction between the APCs and B cells. Plus, B cells are APCs themselves.

All adaptive immune responses must go via activated T cells, since the process of T cell activation is the essential step which includes all the main safeguards to prevent an inappropriate immune response. T cells are the only cells capable of recognising HLA - the point where the HLA presents the antigen to the TcR is the point of recognition.

In addition to Tristian's first post, one of the main actions of antibodies is opsonisation. The Fab (or variable) regions bind the antigen, while the Fc (or conserved) region can be bound by Fc receptors on phagocytes and other immune cells/proteins.

So:
1. Fab portions of antibody binds foreign cell
2. Phagocyte's Fc receptor binds antibody's Fc portion
3. Foreign cell ingested and phagocytosed.

Antibodies can also opsonise complement, which is a collection of proteins which form holes in cell walls. This results in cell death by accelerated osmosis (the water within the cell will pour out these holes). In essence, opsonised antibodies act like a tag for the innate immune system.

In the context of organ transplantation, both cellular (killer T cells and Natural Killer Cells) and antibody responses can result in organ rejection. Antibody responses are the cause for hyperacute rejection. Acute and accelerated rejection can be antibody and/or cellular mediated. Chronic rejection is a massive combination of things, including immune response, immunosuppressant compliancy, scarring from the initial surgery taking it's toll over time and many other things we don't really understand.