Unit 1: Overview of the Immune System and B-Cell Activation

1. Overview of the Immune System

The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful invaders (pathogens) like bacteria, viruses, fungi, and parasites.

The immune system is broadly divided into two main branches: innate immunity and adaptive immunity.

[Image of innate vs adaptive immunity chart]

a) Innate Immunity

• Also known as: Non-specific immunity.
• Response Time: Immediate (minutes to hours).
• Specificity: Non-specific. It recognizes general patterns shared by many pathogens (e.g., bacterial cell walls).
• Memory: No. The response is the same every time it encounters the pathogen.
• Key Components:
  • Physical Barriers: Skin, mucous membranes.
  • Chemical Barriers: Stomach acid, lysozyme (in tears/saliva).
  • Cellular Components: Phagocytes (e.g., Macrophages, Neutrophils), Natural Killer (NK) cells.
  • Inflammation: Redness, heat, swelling, and pain at the site of infection.

b) Adaptive Immunity

• Also known as: Acquired or specific immunity.
• Response Time: Slow (days to weeks) on the first exposure.
• Specificity: Highly specific. It can recognize and target unique molecules (antigens) on specific pathogens.
• Memory: Yes. It "remembers" the pathogen, leading to a faster and stronger response upon re-exposure. This is the principle behind vaccination.
• Key Components:
  • Lymphocytes: B-lymphocytes (B-cells) and T-lymphocytes (T-cells).
  • Antibodies: Proteins produced by B-cells.

2. Humoral and Cellular Immune Responses

The adaptive immune system itself has two arms:

a) Humoral Immune Response

• Mediated by: B-lymphocytes (B-cells).
• Main Target: Extracellular pathogens (e.g., bacteria floating in the blood or lymph).
• Mechanism: When a B-cell is activated, it differentiates into a plasma cell. Plasma cells are "antibody factories" that secrete large amounts of antibodies into the body's fluids ("humors").
• Function: Antibodies bind to pathogens, "tagging" them for destruction by phagocytes or neutralizing them directly.

b) Cellular (Cell-Mediated) Immune Response

• Mediated by: T-lymphocytes (T-cells).
• Main Target: Intracellular pathogens (e.g., cells infected with viruses) and abnormal cells (e.g., cancer cells).
• Mechanism:
  • Helper T-cells (CD4+): "Generals" of the immune system. They activate other immune cells (B-cells and cytotoxic T-cells).
  • Cytotoxic T-cells (CD8+): "Soldiers." They directly find and kill infected or abnormal host cells.

[Image of humoral vs cell-mediated immunity]

3. Immune Components

a) B-lymphocytes (B-cells)

B-cells are born and mature in the bone marrow. Their primary role is to produce antibodies. Each B-cell is covered in B-cell receptors (BCRs), which are essentially membrane-bound versions of the specific antibody it can produce.

b) T-lymphocytes (T-cells)

T-cells are born in the bone marrow but mature in the thymus (hence "T"-cell). They do not produce antibodies.

• T-Cell Receptors (TCRs): T-cells use TCRs to recognize antigens. Unlike antibodies, TCRs cannot "see" free-floating antigens. They can only recognize antigens that are "presented" to them on the surface of another cell by an MHC molecule (see Unit 3).

c) Structure of Immunoglobulins (Antibodies)

Immunoglobulins (Ig), or antibodies, are Y-shaped proteins secreted by plasma cells.

[Image of antibody structure]

Key features:

• Four Polypeptide Chains: Two identical Heavy (H) chains and two identical Light (L) chains, held together by disulfide bonds.
• Constant (C) Region: The "stem" of the Y. This part is the same for all antibodies of the same class (e.g., IgG, IgM). It determines the antibody's function (e.g., how it will be destroyed).
• Variable (V) Region: The "tips" of the Y's arms. This region is highly variable and forms the antigen-binding site. This is what gives the antibody its specificity.

There are 5 classes (isotypes) of immunoglobulins: IgG, IgA, IgM, IgE, IgD (Mnemonic: GAMED).

4. B-Cell Activation

This is the process by which a B-cell is triggered to become an antibody-secreting plasma cell.

a) Antibody Production

1. Antigen Binding: A pathogen's antigen binds to the specific B-cell receptor (BCR) on a naive B-cell.
2. Internalization: The B-cell engulfs the pathogen and presents pieces of it on its surface (using MHC-II molecules).
3. T-cell Help: A Helper T-cell (that recognizes the *same* antigen) binds to the B-cell and releases chemical signals (cytokines).
4. Activation & Differentiation: This "co-stimulation" activates the B-cell, causing it to divide rapidly (clonal expansion). Most of these new cells become plasma cells that start producing and secreting massive amounts of the specific antibody. Some become long-lived memory cells.

[Image of B-cell activation]

b) Class Switching

• Definition: The process that allows a B-cell to change the *class* (isotype) of antibody it produces, while keeping the *same* antigen specificity.
• Mechanism: The B-cell changes the Constant (C) region of its heavy chain gene, but keeps the Variable (V) region.
• Example: A B-cell almost always produces IgM first. Later, with help from T-cells, it can "switch" to producing IgG, IgA, or IgE, which have different functions.

c) Affinity Maturation

• Definition: A process that fine-tunes the antibodies to make them bind *better* (with higher affinity) to the antigen.
• Mechanism: During clonal expansion, the B-cell's antibody genes undergo rapid, random point mutations (a process called somatic hypermutation).
• Selection: B-cells that, by chance, produce a *stronger-binding* antibody are selected to survive and multiply. Those with weaker binding die off.

d) Heavy Chain Gene Transcription

This is the fundamental process of gene expression that allows the B-cell to produce its antibody proteins. The B-cell transcribes the rearranged heavy chain gene (DNA) into messenger RNA (mRNA). This mRNA is then translated by ribosomes into the heavy chain protein. The same happens for the light chain. This process is heavily regulated and increases dramatically when the B-cell becomes a plasma cell.