B Cell Activation and Immunological Memory

Vocabulary flashcards detailing B cell activation, receptor signaling pathways, differences between thymus-dependent and thymus-independent antigens, and characteristics of primary versus secondary antibody responses.

Thymus-dependent (TD) antigens

Antigens, typically soluble proteins, that require direct contact with T helper ($\text{T}_{\text{H}}$) cells to induce a humoral immune response, leading to isotype switching, affinity maturation, and memory cell generation.

Thymus-independent (TI) antigens

Antigens that activate B cells directly through pattern recognition receptors or BCR cross-linking without being displayed on MHC molecules or requiring T helper cell assistance.

Type 1 thymus-independent antigens (TI-1)

TI antigens, such as lipopolysaccharide (LPS) on gram-negative bacteria, that can act as mitogens and induce polyclonal B cell activation at high doses.

Type 2 thymus-independent antigens (TI-2)

TI antigens consisting of highly repetitive molecules, such as bacterial flagella or capsular polysaccharides, that activate B cells by cross-linking the B-cell receptor.

B-2 B cells

The predominant population of B cells in the body, which require thymus-dependent antigens and T-cell interaction for full activation.

Ig-α/Ig-β

Molecules associated with membrane-bound immunoglobulin that possess longer cytoplasmic tails to enable signal transduction via tyrosine kinases upon antigen binding.

ITAM

Immunoreceptor tyrosine-based activation motif; a structural sequence critical for transducing activation signals in B-cell receptor complexes.

Mitogens

Substances that induce non-specific, polyclonal B cell activation, bypassing the requirement for T helper cells.

Primary immune response

The immune response to a first exposure to an antigen, characterized by naive B cells, a lag period of $4-7\text{ days}$, peak response at $7-10\text{ days}$, lower antibody affinity, and early predominance of IgM.

Secondary immune response

The rapid antibody response following subsequent antigen exposures, driven by memory B cells, featuring a $1-3\text{ day}$ lag period, a peak magnitude $100-1000\text{ times}$ higher than the primary response, increased affinity maturation, and predominance of IgG.

Germinal centers

Structures formed in lymph nodes $7-10\text{ days}$ after initial exposure to a thymus-dependent antigen, where somatic hypermutation, affinity maturation, class switching, and generation of memory and plasma B cells occur.

Somatic hypermutation

A mutation process occurring within germinal centers that drives affinity maturation, yielding B cells with higher affinity for a specific antigen.

Class switching

The process directed by cytokines and CD40/CD40L interactions that changes the constant region of antibodies produced by B cells from IgM to other isotypes like IgG.

X-linked hyper-M syndrome

A condition in which T helper cells fail to express CD40L, resulting in patients who produce only IgM antibodies and lack germinal centers and memory B cells.

Complement protein C3 breakdown product

A molecule that provides the second signal required for naive B cell activation following initial antigen recognition and receptor cross-linking.