An important role of natural autoantibodies (NAAs) as potential therapeutic agent historically arises from applications of intravenous immunoglobulin (IVIG) fraction isolated from serum of human healthy adult subjects. IVIG is a sterile product used for non-specific therapy in neurology, hematology, immunology, nephrology, and dermatology [
1]. In dermatology, IVIG is used to treat Kawasaki disease (vasculitis), a disorder recently found to be associated with COVID-19 in children [
2]. The most notable results for the usage of IVIG therapy were achieved for treatment of several neurologic disorders including traumatic brain injury [
3] and multiple sclerosis [
4]. Substantial decrease in plasma level of pathogenic Aβ
42 peptides were also observed in IVIG-treated patients with Alzheimer’s disease [
5]. The current understanding of the mechanism of action of IVIG is not well understood and mechanistically connected with the deactivation of antigen-presenting cells such as macrophages and dendritic cells via Fc receptors (FcR) in an antigen-independent manner [
1]. It is also suggested the therapeutic action is mostly mediated by IgG antibodies that represent ~90-95% of all immunoglobulins in the serum of healthy donors. However, concentrations of other isotypes of immunoglobulins such as IgA and IgM significantly vary from donor to donor [
1]. In healthy human subjects IgM and IgA represent on average less than 5-10% of all reaching the peak by the age of 30-40, following a sharp decline during aging [
6]. Most of IgM and IgA isotypes in the serum of normal subjects represent NAAs, which have broad spectrum specificity and are often self-reactive binding to carbohydrates and lipids on the surface of damaged or apoptotic cells. It is suggested that these IgM or IgA immunoglobulins recognize known or unknown self-antigens and mediate therapeutic effect via of Fc portion of antibodies that bind recently acknowledged FcRs for IgM and low affinity for IgA (known as FcµR) and both IgM and IgA (known as FcαµR) [
7] and by binding of Fab portion of antibodies to self-antigen on the surface of damaged or normal cells [
2]. Particularly, it was shown that self-reactive IgM could bind to the surface of oligodendrocytes and their progenitors (cells that comprise myelin sheath in the central nervous system (CNS)) and promote re-myelination after CNS injury acting via FcμR and FcαμR [
8]. The antigen specificity of these antibodies is still not well characterized, and they are usually present in the serum of healthy donors in low concentrations, which explains the limited therapeutic efficacy of IVIG. To understand the specificity and properties of therapeutic antibodies in IVIG, it is important to understand the basic biology of specific B cell clones that produce these therapeutic self-reactive IgM or IgA within the CNS. During development, most NAAs are produced by a specific type of B cells called B1 B cells [
9]. After birth, the percentage of B1 B cells in the peripheral blood gradually declines over time reaching a minimum level during the aging process similar to the level of NAAs [
6,
10]. During most of adulthood, B1 B cells remain the main source of NAAs that have therapeutic potential in humans [
11]. In this review, we will discuss possible antigen specificity and mechanism of action of NAAs that can activate and fix complement on the surface of neuronal cells to influence the function of neurons including their synaptic activity, response to neurologic insult, survival, and repair.