Hypersensitivity vs. Acute Rejection

[acciddropping]

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In Wikipedia, it says acute transplant rejection is Type2 HS and chronic transplant rejection is type 4 HS - It does not make sense to me much - can you explain why so? In reading through FA, I saw that

1) B cells are involved in a) hyperacute (which makes sense to me, it's preformed AB of donors occluding the grafted vessles), and b) humorally mediated in acute and chronic organ rejections.

2) T cells are involved in acute and chronic organ rejections.

So, now if acute organ rejection is of both humoral and cell mediated, I would think that there would be tissue specific ABs are made via Th2 pathway. So, I can see that it's Type2 HS. But, for Chronic organ rejection, if the humoral immunity is involved, then why it is a type 4? Didn't FA say that Type4HS does not involved Antibodies? (Edit: now that I think about it, it causes fibrosis and delayed response, so that's via the T cells and macrophages and that's why it's type 4)

I thought I understood hypersenstivity stuff but maybe not well.

Type 1- allergic and antibody mediated IgE.
Type 2- cytotoxic (specific organs are targeted and destroyed)
Type 3 - Immune complex (via exogenous pathogens or systemic issues)
Type 4 - No antibodies involved, mediated by T cells.

 

[Enzymes]

Your understanding is totally fine. The point is, you are trying to equate Type IV HS to chronic rejection, which is obviously not correct. Chronic rejection can contain a Type IV hypersensitivity, but there can also be antibodies involved. You are activating CD4 T cells (by displaying graft antigens on antigen presenting cells). CD4s can activate macrophages and mediate damage via Type IV, or they can activate B cells to produce antibodies that then cause damage to the graft.

 

[acciddropping]

Your understanding is totally fine. The point is, you are trying to equate Type IV HS to chronic rejection, which is obviously not correct. Chronic rejection can contain a Type IV hypersensitivity, but there can also be antibodies involved. You are activating CD4 T cells (by displaying graft antigens on antigen presenting cells). CD4s can activate macrophages and mediate damage via Type IV, or they can activate B cells to produce antibodies that then cause damage to the graft.

So, as with acute transplant rejection, can you say it's Type 2? FA does not say it one way or the other. Thanks.

 

[acciddropping]

So, as with acute transplant rejection, can you say it's Type 2? FA does not say it one way or the other. In FA, it says that "transplant rejections" are under type 4. Thanks.

 

[Enzymes]

Well, acute can be humoral (B cell) or cellular (T cell). If antibodies were causing the damage to the cells, I would call that a Type 2 HS. Take home point is this: don't memorize what is associated with what. Instead, understand each hypersensitivity reaction and see what you are dealing with in the question. If antibodies are involved, it would be Type 2. If T cells/macrophages are involved, you should be thinking Type IV.

 

[Psai]

Well, acute can be humoral (B cell) or cellular (T cell). If antibodies were causing the damage to the cells, I would call that a Type 2 HS. Take home point is this: don't memorize what is associated with what. Instead, understand each hypersensitivity reaction and see what you are dealing with in the question. If antibodies are involved, it would be Type 2. If T cells/macrophages are involved, you should be thinking Type IV.

Do you mind explaining the different kinds of rejection? I understand type 1 but for some reason the other ones aren't as obvious to me.

 

[thehundredthone]

The kinds of rejection or hypersenstivity? Type 1-4 are hypersensitivity types, rejection is classified as hyperacute, acute, and chronic.

Hyperacute rejection occurs due to preexisting antibodies against antigens present in the donor tissue, e.g. ABO. Very fast onset, initial vascularization doesn't even occur. Mechanism is largely a type 2 hypersensitivity.

Acute rejection occurs due to humoral and/or T cell mediated immune reaction against antigens present in the donor tissue. Fast onset. The difference between hyperacute and acute rejection may sometimes just be that of the time of onset rather than the mechanism. Mechanism is largely type 2.

Chronic rejection occurs due to repeated humoral and/or T cell mediated immune reaction against the donor tissue. Slow "onset" since it involves multiple smaller episodes over the course of time. The fibrosis aspect is considered to be a type 4 hypersensitivity.