Helper T cellThe T cell that carries CD4. It directs other immune cells.
T cell receptor (TCR)The T cell's own receptor. It binds antigen held in an MHC molecule.
Antigen-presenting cellA cell that shows pieces of antigen to T cells.
MHC class II moleculeHolds a piece of antigen out for the T cell to check.
Antigen (peptide)A short piece of protein from the invader.
CD4A co-receptor on helper T cells. It binds the MHC class II molecule.
CytokinesSmall signal proteins the T cell gives off to other cells.
In the thymus 5
Thymus cortexThe outer zone, packed with young T cells.
Thymus medullaThe inner zone, where nearly mature T cells finish.
Double-negative thymocyteA young T cell with neither CD4 nor CD8 yet.
Double-positive thymocyteA young T cell with both CD4 and CD8.
Naive T cellA mature T cell that has not yet met its antigen.
How this list was compiled: Every name checked on 2026-10-01 and kept only where both of these use it. (1) Wikipedia, text only: T cell, T-cell receptor, CD4, MHC class II, Antigen-presenting cell, T helper cell, Thymocyte and Thymus, https://en.wikipedia.org/wiki/T_cell and https://en.wikipedia.org/wiki/Thymocyte . (2) Two immunology textbooks, read as passages found by archive.org's full-text search: Parham, The Immune System, https://archive.org/details/immunesystem00pete and https://archive.org/details/immunesystem0000parh , and Janeway and others, Immunobiology, https://archive.org/details/immunobiology00cjan : a T-cell receptor recognizes a peptide bound to an MHC molecule; the CD4 co-receptor of helper T cells binds the MHC class II molecule on an antigen-presenting cell; CD4 T cells secrete cytokines; the thymus has an outer cortex and an inner medulla; double-negative and double-positive thymocytes; naive T cells. No OpenStax page was read. Licence: own work
The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work
How it works
This simple T cell diagram has two drawings. The first shows a helper T cell meeting an antigen-presenting cell. The second shows where T cells mature, in the thymus.
Read the full guide
A T cell activation diagram
A T cell cannot see antigen on its own. An antigen-presenting cell holds a piece of it, a peptide, in an MHC class II molecule. The T cell receptor binds the two together, and CD4 steadies the grip. The helper T cell then gives off cytokines.
Growing up in the thymus
New cells arrive from the bone marrow. Under the capsule they are double-negative thymocytes, with neither CD4 nor CD8. Deeper in the cortex they carry both and are double-positive. Cells that pass selection keep one, finish in the medulla and leave as naive T cells.
The molecules are drawn huge so you can tap them; a real cell carries thousands.
The quiz accepts TCR, APC, peptide and MHC II.
Questions and answers
What are the stages involved in T cell development?
A cell from the bone marrow enters the thymus, becomes a double-negative thymocyte, then a double-positive one. After positive and negative selection it keeps either CD4 or CD8 and leaves the thymus as a naive T cell.
Where do T cells mature?
In the thymus, an organ in the upper chest. They are made in the bone marrow and move to the thymus to mature, which is where the T in their name comes from.
What does the T cell receptor bind?
A short piece of antigen, a peptide, held in an MHC molecule on another cell. On a helper T cell, CD4 binds the same MHC class II molecule beside it.
What kinds of T cell are there?
The main kinds are helper T cells, which carry CD4; cytotoxic T cells, which carry CD8; regulatory T cells; and memory T cells.