What is the primary difference between how T cells and B cells recognize antigens?

B cells can bind to free-floating antigens in the body fluids. T cells can only recognize antigens that have been processed and 'presented' on the surface of an antigen-presenting cell (APC).

What is the role of cytokines released by T helper cells?

Cytokines are chemical signaling molecules that stimulate B cells to divide and differentiate into plasma cells. They also enhance the activity of phagocytes to destroy pathogens more effectively.

How do T killer cells eliminate pathogens?

T killer cells bind to antigens on the surface of infected host cells. They then release chemicals that cause the infected cell to lyse (burst), destroying both the host cell and the pathogen inside.

What is a common misconception regarding T cells and antibodies?

A common error is thinking T cells produce antibodies. In reality, T cells coordinate the response or kill cells directly; only B cells (specifically plasma cells) produce and secrete antibodies.

Why is mitosis essential in the T cell response?

Mitosis allows for clonal expansion, creating a large population of T cells that all have the exact same receptor. This ensures the immune response is specifically targeted and powerful enough to overcome the infection.

What defines an Antigen-Presenting Cell (APC)?

An APC is any cell (like a macrophage or an infected body cell) that displays foreign antigens on its surface membrane. This display is necessary to trigger the activation of T lymphocytes.

What happens to the T cell count during an HIV infection?

HIV specifically infects and destroys T helper cells. As the T helper cell count drops, the immune system loses its ability to activate B cells and T killer cells, leading to immunodeficiency.

Define 'Clonal Expansion' in the context of T cells.

Clonal expansion is the rapid proliferation of an activated T cell via mitosis. It results in a large number of genetically identical cells, all capable of recognizing the same specific antigen.

When does a T cell differentiate into a memory cell?

Differentiation into memory cells occurs during the clonal expansion phase following activation. These cells do not participate in the immediate fight but remain in the blood for future protection.

T Cell Response | Pearson Edexcel A-Level Biology

Q: Where do T cells originate and where do they mature?

T cells originate in the bone marrow from stem cells. They migrate to the thymus gland to complete their maturation, where they develop their specific T cell receptors.

6. Immunity, Infection and Forensics

T Cell Response

Summary

The T cell response is a critical component of the adaptive immune system, involving specialized lymphocytes that recognize and respond to specific antigens. Unlike B cells, T cells must have antigens presented to them by other cells, leading to a coordinated cellular response that includes the destruction of infected host cells and the activation of other immune pathways.

1. Definition & Core Concepts

T lymphocytes, commonly known as T cells, are a type of white blood cell that plays a central role in the specific (adaptive) immune response.

They are produced from stem cells in the bone marrow but migrate to the thymus gland to complete their maturation process, which is the origin of the 'T' in their name.

Each mature T cell possesses unique T cell receptors (TCRs) on its surface membrane, which are structurally similar to antibodies and specific to a single type of antigen.

Diagram showing the maturation of a T cell from an immature state to a mature state with specific surface receptors.

2. Activation & Antigen Presentation

T cells cannot recognize 'free' antigens circulating in the blood; they only become activated when an antigen is displayed on the surface of an Antigen-Presenting Cell (APC).

Common APCs include macrophages that have engulfed a pathogen, or host cells that have been infected by a virus and are displaying viral fragments on their membrane.

Activation occurs when the specific TCR on the T cell binds to the complementary antigen-MHC complex on the surface of the APC.

3. Clonal Expansion & Differentiation

Once activated, the T cell undergoes clonal expansion, a process where it divides rapidly by mitosis to produce a large population of genetically identical clones.

These clones all possess the exact same TCR, ensuring the entire population is targeted toward the specific pathogen that triggered the initial response.

The expanded population then differentiates into three functional subtypes: T helper cells, T killer cells, and T memory cells.

4. Functional Subtypes of T Cells

5. Key Distinctions

6. Exam Strategy & Tips