BIOL 251 Chapter 18 Study points

Humoral immunity

Immunity mediated by B cells that produce antibodies to target pathogens in extracellular fluids.

Cellular immunity

Immunity involving T cells, particularly cytotoxic T cells, that target and destroy infected or abnormal cells

Antigens

Molecules that provoke an immune response; typically proteins or polysaccharides on pathogens.

Epitopes (antigenic determinants)

Specific parts of the antigen that antibodies or T-cell receptors recognize and bind to.

What molecules are the best antigens

Large, complex proteins with multiple epitopes; polysaccharides

Antibody structure

Y-shaped molecule with 2 heavy and 2 light chains, variable regions (for antigen binding), and a constant Fc region.

Functions of antibodies

• Neutralization

• Opsonization

• agglutination

• complement activation

• antibody-dependent cell-mediated cytoxicity

How do antibodies act as opsonins

they bind to antigens and enhance phagocytosis by marking pathogens for ingestion by phagocytes.

What are the 5 classes of antibodies

• IgG – most abundant in blood; crosses placenta; long-lasting

• IgA – mucosal surfaces (tears, saliva, secretions)

• IgM – first antibody produced; large pentamer

• IgE – involved in allergies and parasite defense

• IgD – role not well understood; found on immature B cells

T-dependent

Require helper T cells for B cell activation (stronger response, memory)

T-independent

Activate B cells directly (usually polysaccharides; weaker response, no memory).

Antibody response

1. Antigen recognition by B cell

2. B cell activation (with or without T helper cell)

3. Proliferation and differentiation into plasma cells

4. Antibody production

5. Memory B cell formation (in T-dependent responses)

Primary antibody response

First exposure to antigen → slower response, mostly IgM, lower antibody levels.

Secondary antibody response

Re-exposure → rapid and stronger response, mostly IgG, due to memory B cells.

Memory B cells

Long-lived B cells formed after infection or vaccination that quickly respond to future exposures to the same antigen.

MHC Class l

• Found on all nucleated cells; present endogenous (intracellular) antigens to CD8+ cytotoxic T cells.

MHC Class ll

• Found on APCs; present exogenous antigens to CD4+ helper T cells.

Antigen presentation

Process of displaying antigen fragments on MHC molecules for recognition by T cells.

What are antigen-presenting cells (APCs)?

Dendritic cells, macrophages, and B cells.

Helper T cells (CD4+)

Activate B cells, macrophages, and cytotoxic T cells.

Cytotoxic T cells (CD8+)

Kill virus-infected or cancerous cells

What is the role of helper T cells in antibody response?

Provide cytokines and signals to fully activate B cells during T-dependent responses.

How do cytotoxic T cells work?

Recognize infected cells via MHC I, then release perforin and granzymes to kill the cell

CD4

• Helper T cells (interact with MHC II)

CD8

• Cytotoxic T cells (interact with MHC I)

Why must immune responses be controlled?

to avoid damage to host tissues (autoimmunity or excessive inflammation).

How do superantigens work?

Nonspecifically activate many T cells, causing massive cytokine release → toxic shock.

Natural active

• Infection (long-lasting)

Natural passive

• Maternal antibodies (short-term)

Artificial active

vaccination

Artificial passive

Antibody injection (e.g., antivenom)

How do vaccines stimulate immunity?

Introduce antigen in a non-pathogenic form → stimulate immune memory without causing disease.

What is herd immunity?

When enough of a population is immune (via vaccination or infection), it protects even those who aren’t immune.