Cell-Mediated Immunity and T Cells, BIO – Crash Course Immunology Part 3 – Study Notes
Source: Crash Course Immunology, Part 3 (YouTube / Studocu Handout)
Tags: cell-mediated immunity, T cells, helper T cells, cytotoxic T cells, MHC class I, MHC class II, antigen-presenting cells, thymus, cytokines, suppressor T cells, regulatory T cells, apoptosis, granzymes, perforins
Difficulty: Intermediate
Prerequisites: Part 1 (innate immunity, phagocytes, MHC class I) and Part 2 (adaptive immunity, B cells, antigens, antibodies). You should understand how phagocytes break down pathogens and how B cells respond to antigens before reading this.
When pathogens get inside your cells (viruses hijacking cellular machinery, or cells turning cancerous), antibodies circulating in fluids cannot reach them. That is where cell-mediated immunity steps in. T cells are the adaptive immune system's answer to intracellular threats. They work alongside the innate system's antigen-presenting cells and coordinate the entire immune response through chemical signals called cytokines. This part brings together everything from Parts 1 and 2 into a unified picture of how your immune system detects, signals, and destroys threats.
T cells mature in the thymus and respond to antigens displayed on MHC molecules. Helper T cells coordinate the immune response by releasing cytokines. Cytotoxic T cells destroy infected or cancerous cells by triggering apoptosis. Regulatory (suppressor) T cells dial the response back down once the threat is cleared. Both innate and adaptive systems work together through antigen presentation on MHC class I and class II molecules.
Cell-mediated immunity
The branch of adaptive immunity that uses T cells to target and destroy body cells that have been infected by a virus or have become cancerous. When cells are breached, a cell-mediated immune response happens.
In simple terms, this is the immune system going after your own cells that have been hijacked.
T cells (T lymphocytes)
White blood cells made in the bone marrow that mature in the thymus. They are the main players in cell-mediated immunity. T cells cause inflammation, activate macrophages, stimulate other T cells, and mediate the overall immune response.
Thymus
A gland located behind the sternum where T cells mature and learn to distinguish self from non-self. T cells that react to the body's own tissues are eliminated here.
Helper T cells (CD4+ T cells)
T cells that cannot kill directly but coordinate the immune response. They activate cytotoxic T cells, macrophages, and B cells. They release cytokines to amplify the response.
Think of them as the generals who give orders but do not fight on the front line.
Cytotoxic T cells (CD8+ T cells / killer T cells)
T cells that directly kill infected or cancerous cells. They roam the blood and lymph looking for cells displaying abnormal antigens on MHC class I, and trigger apoptosis using granzymes and perforins.
Cytokines
Chemical signalling molecules released by helper T cells (and other immune cells). They amplify the immune response by activating other T cells, B cells, and macrophages, and recruiting more immune cells to the site.
In simple terms, cytokines are the alarm bells that get every part of the immune system moving.
MHC class I (Major Histocompatibility Complex class I)
A protein found on the surface of virtually all nucleated cells. Healthy cells display self-antigens on MHC I, showing the immune system they are normal. Infected or cancerous cells display abnormal antigens on MHC I, flagging them for destruction by cytotoxic T cells.
MHC class II (Major Histocompatibility Complex class II)
A protein found on the surface of antigen-presenting cells (macrophages, dendritic cells, B cells). These cells engulf pathogens, break them down, and display the pathogen's antigens on MHC II for helper T cells to recognise.
Self-antigens
Short chains of amino acids based on proteins synthesised inside healthy cells. They are displayed on MHC class I to signal that a cell is healthy. Also called endogenous antigens.
Antigen-presenting cells (APCs)
Cells that engulf pathogens, process them, and display the pathogen's antigens on their MHC class II molecules. Examples include macrophages, dendritic cells, and B cells.
Granzymes and perforins
Enzymes released by cytotoxic T cells. Perforins punch holes in the target cell's membrane. Granzymes enter through those holes and trigger apoptosis (programmed cell death) from inside the cell.
Memory T cells
T cells that persist after an infection is cleared. On re-exposure to the same antigen, they mount a faster, stronger response, just as memory B cells do in humoral immunity.
Regulatory T cells (suppressor T cells)
A type of effector T cell that releases inhibiting cytokines to decrease and eventually shut down the immune response once the threat is cleared. They prevent the immune system from overreacting or attacking healthy tissue.
Antigen presentation: the bridge between innate and adaptive
Both the innate and adaptive systems use antigen presentation. Phagocytes (macrophages, dendritic cells) break down a pathogen and display fragments of it as proteins called antigens on their surface.
MHC class I is found on all nucleated cells. Healthy cells display self-antigens (short amino acid chains from normal proteins synthesised inside the cell). This signals "I am healthy" to patrolling T cells.
MHC class II is found on antigen-presenting cells (APCs): macrophages, dendritic cells, and B cells. APCs engulf foreign material, break it down, and present the pathogen's antigens on MHC II for helper T cells to inspect.
T cell development
T cells are made in the bone marrow but mature in the thymus.
During maturation, T cells that would attack the body's own tissues are eliminated (a process called negative selection). Only T cells that correctly ignore self-antigens and respond to foreign antigens survive.
Helper T cells
Helper T cells cannot kill infected cells directly, but they are essential coordinators.
Each helper T cell has receptors that bind to one particular combination of MHC class II and antigen. This specificity means each helper T cell responds to one type of threat.
When a helper T cell binds to an APC displaying its matching antigen on MHC II, it activates and begins cloning itself, producing effector helper T cells and memory T cells.
Activated helper T cells raise the alarm by releasing cytokines.
Cytokines activate other helper T cells (positive feedback loop), cytotoxic T cells, macrophages, and B cells. They amplify the entire immune response.
Cytotoxic T cells
Cytotoxic T cells roam the blood and lymph, scanning cells for abnormal antigens on MHC class I.
When they find a cell displaying a foreign or abnormal antigen on MHC I (indicating viral infection or cancer), they trigger apoptosis by releasing granzymes and perforins.
Perforins punch holes in the target cell's membrane. Granzymes enter through those holes and activate the cell's self-destruct programme.
B cell activation (linking humoral and cell-mediated immunity)
B cells also function as APCs. A B cell engulfs a substance, breaks it down, and presents the antigen on its MHC class II.
The B cell then pauses, waiting for a helper T cell to bind and deliver cytokines.
Once it receives that signal, the B cell activates fully, producing antibodies and launching the humoral immune response.
This is why helper T cells are so critical: without them, B cells cannot mount an effective antibody response to most antigens.
Regulatory (suppressor) T cells
Once the infection is cleared, the immune response must be shut down to prevent damage to healthy tissue.
Regulatory T cells are a type of effector T cell that releases inhibiting cytokines, which decrease and eventually stop the immune response.
They act as the brakes on the immune system.
HIV destroys helper T cells (specifically CD4+ cells). This is why it is so devastating: without helper T cells, neither the cell-mediated nor the humoral immune response can function properly. AIDS is the result of the immune system collapsing because its coordinator has been removed.
Organ transplant rejection happens because the recipient's T cells recognise the donor organ's MHC molecules as foreign. Immunosuppressive drugs work by dampening T cell activity, which is why transplant patients are more vulnerable to infections.
Students often confuse MHC class I and class II. Class I is on all nucleated cells and displays self-antigens (or viral antigens if infected); cytotoxic T cells read it. Class II is only on antigen-presenting cells (macrophages, dendritic cells, B cells) and displays processed pathogen fragments; helper T cells read it.
Students sometimes think helper T cells kill infected cells. They do not. Helper T cells coordinate. Cytotoxic T cells do the killing.
Students sometimes believe that B cells activate on their own. For most antigens, B cells require a signal (cytokines and binding) from a helper T cell before they produce antibodies.
Students often forget that regulatory T cells exist. The immune response is not a one-way escalation; it has a built-in off switch.
⚠️ Know the difference between MHC class I (all nucleated cells, read by cytotoxic T cells) and MHC class II (APCs only, read by helper T cells).
⚠️ Be able to trace the activation sequence: APC presents antigen on MHC II to helper T cell, helper T releases cytokines, cytokines activate cytotoxic T cells and B cells.
⚠️ Understand how cytotoxic T cells kill: perforins make holes, granzymes enter and trigger apoptosis.
⚠️ Know that T cells mature in the thymus (not the bone marrow, where they are made).
⚠️ Be clear on the role of regulatory T cells: they release inhibiting cytokines to shut down the immune response.
True or False: Helper T cells directly kill infected cells.
Answer: False. Helper T cells coordinate; cytotoxic T cells do the killing.
Fill in the blank: T cells are made in the bone marrow but mature in the ______.
Answer: Thymus.
True or False: MHC class II is found on all nucleated cells.
Answer: False. MHC class II is found only on antigen-presenting cells (macrophages, dendritic cells, B cells).
Fill in the blank: Cytotoxic T cells kill target cells using ______ and ______.
Answer: Granzymes and perforins.
True or False: Regulatory T cells amplify the immune response.
Answer: False. They release inhibiting cytokines to decrease and shut down the immune response.
Q: Compare the roles of helper T cells and cytotoxic T cells.
A: Helper T cells coordinate the immune response by binding to antigen-presenting cells via MHC class II and releasing cytokines that activate cytotoxic T cells, B cells, and macrophages. Cytotoxic T cells directly destroy infected or cancerous cells by recognising abnormal antigens on MHC class I and releasing granzymes and perforins to trigger apoptosis.
Q: Trace the steps from a macrophage engulfing a pathogen to a cytotoxic T cell killing an infected cell.
A: A macrophage engulfs and breaks down the pathogen, then displays its antigens on MHC class II. A helper T cell with the matching receptor binds to the macrophage's MHC II-antigen complex and activates. The helper T cell releases cytokines, which activate cytotoxic T cells. Cytotoxic T cells then scan the body for cells displaying that pathogen's antigens on MHC class I. When they find an infected cell, they release perforins (to puncture the membrane) and granzymes (to trigger apoptosis).
Q: Why are helper T cells critical for B cell activation?
A: After a B cell engulfs a substance and presents its antigen on MHC class II, it pauses and waits for a helper T cell to bind and deliver cytokines. Without this signal from a helper T cell, the B cell cannot fully activate to produce antibodies and mount a humoral immune response.
Q: What is the function of regulatory (suppressor) T cells, and why is it important?
A: Regulatory T cells release inhibiting cytokines that decrease and eventually shut down the immune response once an infection is cleared. Without them, the immune system would continue attacking, potentially damaging healthy tissue (autoimmune-like damage).
Q: Explain the difference between MHC class I and MHC class II in terms of location, function, and which T cell reads each.
A: MHC class I is on all nucleated cells and displays self-antigens (or foreign antigens if the cell is infected). Cytotoxic T cells read MHC I. MHC class II is only on antigen-presenting cells (macrophages, dendritic cells, B cells) and displays processed pathogen antigens. Helper T cells read MHC II.
Cell-mediated immunity ties directly back to Part 1 (innate immunity). Macrophages and dendritic cells from the innate system serve double duty as antigen-presenting cells, bridging innate and adaptive responses. Without innate phagocytosis, T cells would have nothing presented to them.
This topic also connects to Part 2 (humoral immunity) through B cell activation: helper T cells are the link between cell-mediated and humoral branches. The two systems work as a coordinated whole, not separate departments.
For students studying pathology or clinical medicine, understanding how T cells work is essential for topics like autoimmune diseases (where regulatory T cells fail), immunodeficiency disorders (like HIV/AIDS, which destroys helper T cells), and cancer immunotherapy (which harnesses cytotoxic T cells to target tumours).
Cell-mediated immunity, cellular immunity, T cell, T lymphocyte, helper T cell, CD4+ T cell, cytotoxic T cell, CD8+ T cell, killer T cell, regulatory T cell, suppressor T cell, memory T cell, thymus, T cell maturation, negative selection, MHC class I, MHC class II, major histocompatibility complex, antigen-presenting cell, APC, macrophage, dendritic cell, B cell as APC, antigen presentation, cytokines, granzymes, perforins, apoptosis, programmed cell death, clonal expansion, effector T cell, inhibiting cytokines, immune regulation, Crash Course immunology Part 3