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I have seen the answer to this question which says that T cells do not express MHC II proteins which would make sense.

However, my textbook "The immune system" by Peter Parham disagrees. It says that T cells do gain MHC II receptors after activation.

enter image description here

*Activated T cells express MHC class II molecules, whereas resting T cells do not.

I cannot see why this is needed. MHC II molecules are for CD4 cells (helper T cells).

If the T cell presenting the antigen was a CD4 cell, then that seems useless, because this cell was already the one that could recognise this particular antigen - so why show it to others? The chance that another T cell that could recognise it too would be incredibly small no?

I guess I can see the merit of a CD8 T killer cell to show the antigen, as it could alert other CD4 T cells to come assist (although cytokines seem like a better method...)

So what is the purpose of T cells, in particular CD4, of showing MHC II proteins?

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enter image description here

That's the description in my book which has the same image.
The book is: Janeway's Immunology (9th Edition)

Article discussing MHC Class II expression in humans:
https://doi.org/10.4049/jimmunol.168.2.763

Excerpt from the book:

The processing and presentation of pathogen-derived antigens has two distinct purposes: inducing the development of armed effector T cells, and triggering the effector functions of these armed cells at sites of infection. MHC class I molecules bind peptides that are recognized by CD8 T cells, and MHC class II molecules bind peptides that are recognized by CD4 T cells, a pattern of recognition determined by specific binding of the CD8 or CD4 molecules to the respective MHC molecules (see Section 4-18). The importance of this specificity of recognition lies in the different distributions of MHC class I and class II molecules on cells throughout the body. Nearly all somatic cells (except red blood cells) express MHC class I molecules. Consequently, the CD8 T cell is primarily responsible for pathogen surveillance and cytolysis of somatic cells. Also called cytotoxic T cells, their function is to kill the cells they recognize. CD8 T cells are therefore an important mechanism in eliminating sources of new viral particles and bacteria that live only in the cytosol, and thus freeing the host from infection.

By contrast, MHC class II molecules are expressed primarily only on cells of the immune system, and particularly by dendritic cells, macrophages, and B cells. Thymic cortical epithelial cells and activated, but not naive, T cells can express MHC class II molecules, which can also be induced on many cells in response to the cytokine IFN-γ. Thus, CD4 T cells can recognize their cognate antigens during their development in the thymus, on a limited set of ‘professional’ antigen-presenting cells, and on other somatic cells under specific inflammatory conditions. Effector CD4 T cells comprise several subsets with different activities that help eliminate the pathogens. Importantly, naive CD8 and CD4 T cells can become armed effector cells only after encountering their cognate antigen once it has been processed and presented by activated dendritic cells.

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I believe in the article you provided it is stated that the function of MHC class II molecules on activated CD4+ T cells is still unclear. It is believed that these MHC class II molecules-presenting T cells have potential ability to act as antigen presenting cells and induce immune response by activating other T cells. They are also capable of inducing down-regulatory signals for immune response by inducing anergy in activated T cells and cytotoxic resting T cells. In addition, they have also been reported to transduce intracellular signals into T lymphocytes.

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