U01.02.010 Major functions of B and T cells

B and T cells are central to adaptive immunity, each performing specific roles in response to pathogens. Their combined action ensures that both humoral and cell-mediated immune responses are effective against a wide variety of threats, including infections, tumors, and transplants.

B Cells: Humoral Immunity

Key Functions:

Antigen Recognition and Presentation:
- B cells recognize antigens via B cell receptors (BCR).
- They undergo somatic hypermutation, a process that optimizes the specificity of the BCR for a particular antigen.
Antibody Production:
- B cells differentiate into plasma cells, which secrete immunoglobulins (antibodies) specific to the recognized antigen.
- These antibodies neutralize pathogens and mark them for destruction.
Immunologic Memory:
- Memory B cells persist after an infection.
- These cells enable a faster and more robust immune response upon subsequent exposure to the same antigen.

T Cells: Cell-Mediated Immunity

Key Functions:

CD4+ T Cells:
- Help B cells in antibody production.
- Produce cytokines that recruit phagocytes and activate other leukocytes to enhance the immune response.
CD8+ T Cells:
- Directly kill virus-infected and tumor cells via the release of perforin and granzymes.
- These functions are similar to natural killer (NK) cells.
Hypersensitivity Reactions:
- Delayed cell-mediated hypersensitivity (Type IV) is mediated by T cells.
Organ Rejection:
- T cells are involved in both acute and chronic forms of cellular organ rejection.

Rule of 8:

MHC II × CD4 = 8: CD4+ T cells interact with antigens presented on MHC class II molecules.
MHC I × CD8 = 8: CD8+ T cells interact with antigens presented on MHC class I molecules.

Table: Summary of B and T Cell Functions

Cell Type	Immunity Type	Key Functions
B Cells	Humoral Immunity	– Antigen recognition & presentation – Antibody production – Memory B cell formation
T Cells	Cell-mediated Immunity	– CD4+ T cell assistance – CD8+ cytotoxicity – Hypersensitivity (Type IV) – Organ rejection

Points to Remember:

B cells are primarily responsible for Humoral immunity through antibody production.
T cells mediate cellular immunity, with CD4+ T cells helping in immune coordination and CD8+ T cells directly killing infected cells.
The Rule of 8 simplifies understanding the interaction of T cells with MHC molecules.

Bibliography:

Abbas, A. K., Lichtman, A. H., & Pillai, S. (2018). Basic Immunology: Functions and Disorders of the Immune System. Elsevier.
Murphy, K., Weaver, C. (2016). Janeway’s Immunobiology. Garland Science.
Parham, P. (2014). The Immune System. Garland Science.

USMLE Step 1 Review Series

Curriculum

U01.02.010 Major functions of B and T cells

B Cells: Humoral Immunity

Key Functions:

T Cells: Cell-Mediated Immunity

Key Functions:

Rule of 8:

Table: Summary of B and T Cell Functions

Points to Remember:

Bibliography:

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