T cells recognize a "non-self" target, such as a pathogen, only after antigens (small fragments of the pathogen) have been processed and presented in combination with a "self" receptor called a major histocompatibility complex (MHC) molecule. Cell mediated immunity
There are two major subtypes of T cells: the killer T cell and the helper T cell. In addition there are regulatory T cells which have a role in modulating immune response. Killer T cells
Killer T cells are a sub-group of T cells that kill cells that are infected with viruses (and other pathogens), or are otherwise damaged or dysfunctional. As with B cells, each type of T cell recognizes a different antigen. Killer T cells are activated when their T-cell receptor binds to this specific antigen in a complex with the MHC Class I receptor of another cell. Recognition of this MHC:antigen complex is aided by a co-receptor on the T cell, called CD8. The T cell then travels throughout the body in search of cells where the MHC I receptors bear this antigen. When an activated T cell contacts such cells, it releases cytotoxins, such as perforin, which form pores in the target cells plasma membrane, allowing ions, water and toxins to enter. The entry of another toxin called granulysin (a protease) induces the target cell to undergo apoptosis. T cell killing of host cells is particularly important in preventing the replication of viruses.
They occur at the end of injured nerve fibres as a form of ineffective, unregulated nerve regeneration; it occurs most commonly near a scar, either superficially (skin, subcutaneous fat) or deep (e.g., after a cholecystectomy). They are often very painful. Synonyms include scar neuroma, amputation neuroma, or pseudoneuroma. Mortons neuroma (a mononeuropathy of the foot) is another example of the more general usage of the term neuroma. Some prefer the term "Mortons metatarsalgia", thus avoiding the term neuroma and its association with tumors. Etymology
The stem neuro- originates from the Greek word for nerve (νεῦρον), while the suffix -oma (-ωμα) denotes swelling. The stem does not imply that neuromas necessarily arise from neurons; neuromas generally arise from non-neuronal nerve tissues. The word was originally used to refer to any nerve tumor, but its meaning has evolved. == References ==